My brother, Christian, was a benevolent dictator, though only 11 months older than me; Irish twins, they call it. Early on, we got the idea that it would be sensible to look after one another, and our private mythology of brother and sister as the two faces of a coin, around which so much of our lives has taken shape, was forged before either of us could speak. I recently found a snapshot of the two of us hand in hand, walking ourselves to kindergarten. It's taken from behind, and we don't know we're being watched. Our white kneesocks are pulled very high. Who took it—my mother or my father? What were they thinking as we set off on our own?

The bedtime song my mother sang us was called "Babes in the Woods." We clamored for its lilting melody: "They sobbed and they sighed, and they bitterly cried, and the poor little things, they laid down and died." Here we tucked ourselves side by side under her arm to savor their deliciously awful fate. "And when they were dead, the robins so red/ Brought strawberry leaves and over them spread."

I was very happy to have a mother who didn't want to leave me in the woods (although sometimes she must have, rather badly), but my imaginings never extended to being a mother myself. Press-ganged into babysitting by neighbor parents, I noted their eagerness to leave, how they rattled off emergency numbers while the car keys jingled. Christian and I were terrible sitters, impatient, insincere. The kids knew it. One used to ask accusingly, "What are you doing here, stupid?" Christian gave large doses of baby Tylenol to another hapless little complainer. I quit the job as soon as I got a work permit and could clear plates for $2 an hour plus tips.

From birth, Christian had a sense of flair: In grammar school he'd cut our peanut-butter-and-jellies into crustless tea sandwiches and include a fluted paper plate in our lunch bags, indifferent to mockery. By adolescence, this quality had toughened into a defiant flamboyance, which took breathtaking moral—and sometimes physical—courage to carry off in 1983 New England. Injustices done to him—Neanderthal Brad Crawley throwing Suzy Q's at my beautiful and rare brother!—made my skin burn with caustic fury. But I was dealing with misfitness too, though instead of blazing, as he did, I moped and skulked. He left for college, and the house was horribly quiet. No one to dance around waving a dish towel and singing show tunes while I did the dishes.

But, as it happens, the following year we went off to school together again, the first of many thousand-mile drives in a 1974 Chevy Impala with a Styrofoam cooler of fried chicken in the backseat. I'd bothered to apply to only one college. Had he decided ahead of time, in his imperious way, that I'd follow him to his?

At college, I was delighted and relieved to find that he was loved. Stoners, sorority girls, supercilious professors, and ROTC cadets adored him for his absurdist wit and his air of having trailed a little bit of splendor behind him, like the sharp winter smell that follows you in from outside. Christian taught me how to pull a respectable bong hit, how to find the nerve to fling myself into a pool of conversation, and how to sign up for the right classes—that is to say, those with the least practical application: Introduction to the Art Song, the Seven Hills of Augustan Rome. "Come on, sister, have yourself a ball!" So The Kinks song went; I heard it for the first time on a body-swallowing sofa while getting high with his friends. "Don't be afraid to come dancing, it's only natural."

A job as a book editor eventually took me to London, where Christian had gone to work as a dramatic agent, and after a gap of a decade we once more lived close by. If I felt lonely, I could put a coat on over my nightgown and walk unnoticed through the sleazy all-night carnival of Old Compton Street to hang out at the flat he shared with Mikey, his boyfriend and then husband once England discreetly began to allow civil unions. Christian and I spent our professional lives looking after sometimes emotionally fragile people. It is not easy being a creative person, and the tantrums that were thrown, the vulnerabilities that cracked open and needed to be patched up again, could be wearying and unnerving. Like children who lacked the adorability to seduce you into not minding so much. "The world," Christian said darkly one of those evenings, "has enough people.You and I do not need to add to them." And I was happy to sign this latest treaty of mutual intention with him. Every now and then I'd squint to visualize a time when I'd start craving a child: When I was 30, it would be 32; at 32, I'd be ready by 36; and so on. I was hitching a ride on Zeno's arrow, speeding toward a target I'd never reach.

Meanwhile, there had been a societal swing back to the orthodoxy of motherhood. Serious journalists wrote with anguish of their biological clocks, a phrase I came to hate. All the available cultural artifacts seemed to be telling holdouts like me that if you were a woman, your business was having a baby, and if you didn't, there was something wrong—with your body, meaning you couldn't conceive, or with your mind, meaning you couldn't conceive of it.

"I'd get on with it, if you're going to do it," my gynecologist said, blunt as a speculum. A literary agent with a litter of kids told me over lunch that I'd regret my decision but by then it would be too late, and she smacked her hand down on the tablecloth so hard that our water glasses sloshed. (What decision? What and when had I decided?) A friend held both my reluctant hands, eyes drilling into mine, and said that for her having children was like flicking on the light in a dark room. But the older I get, I thought mutinously, the more I like a bit of dim lighting. At 40, it's easier on the complexion.

In the meantime, Christian and I had taken to pointing to each other.

You do it.

No, you do it.

And we laughed.

So much of being a grown-up is about managing or quelling desires. For food, for drink, for sex, for good times; if you're a woman, I maintain, for ambition. You should not want too much. It is strange, then, to be in a position where society demands you have an appetite for something. And yet here was a rare instance where I was appetite free, and the world seemed to be saying, "You have to want this thing, if only so we can help you work through your feelings about not having it!"

And so I set about trying to try, with the same enthusiasm that I would have brought to cooking a Thanksgiving dinner and sitting down to joylessly chew the whole thing myself. The big joke, after all that worrying in my twenties, is that it isn't so easy to get pregnant later on. And now that I had skidded past 40, I didn't. I wasn't relieved, exactly, but I wasn't sorry, either. I felt with some satisfaction that my body had honorably answered for my whole family this lingering question of whether there would be a next generation of Hodells. I'd done my duty, and now we could all move on. The two kids with their high white socks were now undeniably middle-aged.

One afternoon, Christian emailed to say that he and Mikey had something important to talk to me about. His Important Conversations could be unpredictable and sometimes terrifying: Why He Is the Wrong Boyfriend for You; Your Job Is a Poisoned Chalice; That Lipstick Shade Does Not Flatter. (We all feared the familiar words, "I'm going to say this with love....") We Skyped; I trained my face to look serenely receptive.

But this time it was not about me. The comedy of it! While my family had glanced covertly my way, wondering when I'd get around to marrying, my gay brother had gone and done it. And now he'd visited a clinic in Connecticut to flip through binders of baby mamas. He and Mikey squeezed close so they'd both fit onto my monitor to say that they'd picked an egg donor—a pretty brown-eyed law student of Czech extraction from Rhode Island—and with luck and about $100,000, in a year's time they'd be parents. I hadn't even known they were considering it.

In Connecticut, Mikey and Christian met the law student for 15 nervous minutes before she was wheeled in to have her eggs vacuumed up. They both contributed—I didn't ask, but I imagined it involved specialized magazines in a toilet stall—and the results were eyedroppered onto her eggs. "We've got 13 embryos in the freezer," Christian reported expansively. "You could have one, if you want."

They'd also found a surrogate, the magnificent and sainted Sharla, who lived all the way out in Wichita, Kansas. She was flown to the clinic, and several embryos were implanted. Christian returned to London, and I was visiting him there—I'd moved back to the U.S.—when he got the news of a strong single heartbeat. Sitting in his fishbowl office with his staff clapping around him, he rejoiced and we all cracked each others' spines with hugs like a convention of chiropractors. But when he shut the door, tears glazed his eyes: "I mind that there aren't going to be two to grow up together."

Soon, Sharla emailed ultrasounds in which a little bean could be seen and then not seen, inky and blurred, like an old mezzotint. Christian and Mikey talked baby names for hours. "Now let's do jewels! Ruby. Pearl? Jade." In the end, she was Elsa. I flew to Kansas on her birth to be housekeeper while they figured out how to be parents. Christian was Papa; Mikey was Daddy. But the dot of blood harvested when she was minutes old would show that she was Christian's biological child. "That's mine," he whispered disbelievingly.

It would take a month to get Elsa's documents in order, and they rented a suite in a sort of shantytown for transient executives. Sharla pumped as much breast milk as she could. Bottles of it sat, unsettlingly yellow, in the fridge among our groceries. This generous stranger, no blood of ours, had the most sustained physical relation to Elsa of any of us. She had made her—or rather, she'd allowed Elsa to make herself inside her, spinning her little body from the genetic material of my brother and a woman none of us would ever see again.

Not everyone falls in love with a newborn. That is this auntie's secret. Elsa was a red and wrinkly visitor from outer space, skinny, with a slightly lopsided face and opaque mineral-blue eyes that minutely raked the face of whoever was bent over her with the bottle, searching as her little mouth worked. Things were most definitely going on in there, but who could say what? Her squalls were spasmodic, weak, shuddering, as if her small bones weren't sturdy enough to withstand the gusts of wanting. When I welled up at the noises of grief, Christian snapped, "Are you drunk?"

He snatched her up, swaddled like a canapé, and speed-skated around the living room in his socks, singing Christmas carols as Elsa stared up at him, transfixed. He whisked past me. "There's Tatie Courtney!" This was the shocker. He was a natural father, easy, confident, fearless. How was he allowed to be different from me?

Wichita seemed to be all mall, and we toured one after another in an enormous rented SUV, shopping for the numerous items necessary for a weeks-old baby. Christian was explaining how her life was going to go. "She'll ski, and she'll speak French; she'll play tennis and the piano. Everything else she gets to pick for herself."

The atmosphere in the car shifted a little; I could tell he was working up to something. I glanced over at his profile with the ribbon of Kansas beyond it. His Byronic swoop of hair was clipped like Caesar's now, but he'd grown into his handsome nose, and I thought he looked very distinguished and not at all improbable at the wheel of the big car. "Tell me about the...about the coochy." He couldn't quite get the word out.

"You mean the vagina?" I bit down a laugh. Really, I was thrilled to be asked about a subject I could at last feel learned about. "Think of it as a kind of self-cleaning oven. You don't need to get up in there with any soap or whatnot. It takes care of itself as long as you keep the outside area clean...." And so I went on.

His knuckles tightened on the steering wheel after a time. "Okay, great. I don't think I can hear any more right now." He appeared to be breathing through his mouth. "But thanks. Really helpful."

Poor boy. I realized I didn't know if he'd ever seen a vagina up close, and now he was in charge of making someone feel comfortable about hers. We steered into a consoling Krispy Kreme drive-through with the HOT NOW sign lit up. I realized that it would probably be a decade before he next asked for my advice, when the dreaded menses loomed.

Elsa was no longer than my forearm, and there was just so much turbulence ahead. Girls are born with all the eggs they will ever have, enough to populate a small city. But these start dying off at birth, and only a few hundred of them will ever kick off into the fallopian tubes and mature into the big chance. Women have, I'd guess, about two decades of genuine, galloping fertility. Twelve periods a year, that's 240 shots at making a baby without enlisting a team of professionals and some lottery winnings. Why was I thinking about this already? She was a few weeks old. This was the telescoping nature of human endeavor. All that mad activity— going to parties, falling in love, buying houses, striving at work—could be smashed like a soda can into this flat fact: We have children so they can have children so they can have children. I had a blast of vertigo, as when you look into a puddle and see the stars falling away behind your head.

Elsa got her passport, Sharla's milk dried up, and we all dispersed, exhausted: Mikey and Christian to a wholly altered life, with unrecognizable hours and fears and blisses, and me back to mine, where there was still a sock lying in the middle of the rug and an empty glass in the sink.

I'm no Facebooker, but I started checking in daily to see photos of them settling in, 3,500 miles away. One morning Christian posted, Last day of my paternity leave. Devastated. From this moment on, everything I do is for her and her wonderful daddy.

Here it was: I'd been kicked out of our tiny Narnia. The wardrobe held only coats. The cold stone in my chest was the rightness of what he'd written. In his novel On the Black Hill, Bruce Chatwin describes grown twins: "Because they knew each other's thoughts, they even quarreled without speaking." Now my brother was thinking and feeling things I never would. In college he'd taught me how to speak, but this was something I could never say aloud: Don't leave me behind.

The only recourse was to love this little scrap of a human, and in the first really adult way I would love anyone. Without expectations of returned affection. Without wounded vanity. With foreknowledge of impending boredom, exasperation, even anger. In the understanding that I would sometimes be ridiculous in her eyes. Knowing I did not have the rights of parenthood, I could make no demands of her beyond those any grown-up would make of a child: Hold my hand, we're crossing the street....

When Nathan, my boyfriend of five years now, held Elsa for the first time, he wept—big sparklers caught in his lower lashes, too light to drop. "Not sadness," he said, "just big feeling." Now the decision is made. But the decision is not past. No matter how it came about—was it my procrastination, disinclination, anxiety, self-absorption?—we live with its consequences every day. Nathan is younger than I am, and it's a little odd to be dealing at his age with the question of whether he'll have his own children or not. As long as he chooses to be with me, it will be the latter. I want him to stay, but it is, as they say, a big ask.

I've learned from the work of the primatologist Sarah Blaffer Hrdy that aunts exist in nature. Of course they're everywhere, biologically speaking, but some (marmosets and langurs, I'm looking at you) truly behave as the aunt I want to be, the aunt I'm becoming, and this is called allomothering. They'll feed, groom, and hold a child when they don't have one of their own. So there is a word for what I'm doing, I and all my sisters of the genetic dead end. Whatever I've learned in this life will not stop with me; I'll teach it to her.

From feeling we move to thinking, and then to doing. "If there is a kindness instinct," writes psychologist Adam Phillips, "it is going to have to take onboard ambivalence in human relations. It is kind to be able to bear conflict, in oneself and others; it is kind, to oneself and others, to forgo magic and sentimentality for reality...."

As night falls in the forest, Hansel crowns Gretel Queen of the Woods and sings to her, "I give you the strawberries, but don't eat them all." It is hard, so hard, to let go of a story you've lived by. Brother, good-bye; father, hello. As in fairy tales, there must be a gift at a christening, and this one is my offering: For her, the wild strawberries will be only strawberries, and sweet. "Fraises des bois, Elsa darling," my brother will say. "Try one."

Adapted from Meghan Daum, ed., (Picador © 2015)

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