By TESS SIMPSON

Last updated at 23:27 04 May 2007

Sorry, my sister has mucked up her life, and if her only kidney fails I'm not going to donate one of mine to save her.

When I was ten years old and my sister Philippa was eight, we became "blood sisters".

Sharing the same genetic parents wasn't enough, we decided; we needed to cut our wrists and mingle our blood if we wanted to be real sisters.

Using a rusty penknife of the kind that would have health and safety officers in hysterics these days, we opened our veins, rubbed our wrists together, and swore everlasting love and loyalty until the end of time.

I protected her against all comers in the neighbourhood and at school; in return, she covered for me and took the blame when I transgressed at home.

We shared a room until I was 16, and were so close we joked about living together with our husbands when we grew up.

I pinched her make-up; she broke my favourite doll.

We fought and bickered like any sisters, but the moment anyone threatened either of us, we closed ranks and backed each other to the hilt.

We grew up in a loving, middle-class Roman Catholic family in West Sussex, together with our younger brother, Charles.

Our father, Michael, worked as a contracts manager for an engineering firm; our mother, Jane, was an editor for Jane's Defence books until she died five years ago.

We had a normal, happy childhood; there was never a great deal of money, but there was always plenty of love.

As a child, my sister was always sickly. She had pneumonia 13 times in two years; her lungs collapsed, and one of them never recovered.

Her kidneys also gave her problems, and when she became pregnant with the first of her three sons at the age of 22, one of them finally gave up altogether.

She's now 36, and for the past 14 years, she's limped along with a single fitfully functioning kidney, spending frequent spells in hospital.

We both know that she could lose the use of her remaining kidney at any time, condemning her to a lifetime of dialysis unless she finds a donor.

As her sister, I'd be her best bet for a genetic match.

And yet a few weeks ago, I came to the difficult decision that if she asked me for a kidney, I'd say no.

I know many people's instinctive reaction will be to condemn me as heartless, even cruel.

But the truth is my sister is both feckless and reckless with her health.

She drinks, she's used drugs, and she has three children by three men - all of whom have now been taken into care.

So I'm sorry, but I'm simply not prepared to risk my own health and the future of my own family to bail her out.

Recently, the case of a woman in Cheshire who refused to donate bone marrow to her dying brother became the subject of great debate, with critics from all sides branding her wicked and even somehow unnatural.

Who knows what family secrets lurk behind her seemingly selfish decision?

Without knowing the full facts, we shouldn't rush to judgment.

But asking someone to donate a kidney or bone marrow isn't like wanting to borrow their car for the weekend.

It can be a life-changing decision for the donor too, and needs to be made calmly and clearly, rather than on a wave of sentimental emotion.

Admittedly, that goes against the grain when it comes to family. We all think with our hearts rather than our heads when someone we love is in danger.

But we owe it to all those who depend on us, not just the person in need, to make the right choice - which isn't always the easiest one.

The reason I cannot in all conscience now offer a lifeline to my sister is that since we left home our lives have followed such different paths that - in brutal terms - I am not now willing to risk my life to save hers.

I did better at school than my sister, in part due to her constant illnesses, and at 18, won a place at Oxford. Philippa left school the same year, with a good clutch of O-levels, to take a secretarial job.

At first, her career went well; she was promoted to the company director's PA by the time she was 19.

She was earning good money, while I was struggling to get by on a student grant; generous to a fault, she bought me expensive presents and always paid for dinner when we met up.

But then it all started to go wrong for her. She started a doomed affair with a married man who took advantage of her youth and naivety.

He treated her very badly and broke her heart, and I think many of the things that went wrong for her later in life can be traced to this man.

Not long after he ended their relationship, she suddenly announced her engagement to a young man she'd known only a few weeks.

It was clearly a rebound relationship, but within months, they were married.

We all knew it was going to be a disaster, so no one was very much surprised when they split up less than two years later.

Perhaps she could have put it down to experience and moved on with her life - but then she found out she was pregnant.

Her ex-husband demanded a paternity test, which duly proved the baby wasn't his, and when she was just three months pregnant, she moved in with a new boyfriend. He was initially very supportive, but when the baby was born, he started to resent him.

Thinking he would come round to the idea of fatherhood if he had his own child, she deliberately got pregnant again without telling him. As soon as he found out, he left her.

At 23, she was alone with a 12-monthold son and a newborn baby. She couldn't cope, and social services were called in.

She decided to put the children into care for six months while she got her life back together. Naturally, the family was dismayed, but she started therapy and a course of anti-depressants, and we all rallied round to support her.

The doctors had diagnosed her with a personality disorder.

Her psychiatrist told her not to get involved in another relationship until she was more stable, but once again, she met a new boyfriend, moved in with him, and got pregnant.

The new baby was born just 25 months after her first child - she'd had three boys in just over two years by three different fathers.

This time, the boyfriend did at least stick around, but we all wished he hadn't.

He didn't have a job, and spent much of his time taking drugs and drinking. But my sister seemed head-over-heels in love.

Her two older sons were returned to her, and she got married again and tried to make a home for her young family.

Now it was my turn to help her out financially.

At that point, having worked as a producer at ITN, I was married to a CNN correspondent and forging a career as a writer.

Philippa would call saying she had no food for the children, so I'd drive over and take her to the supermarket to stock up, or pay her rent when she was about to be evicted.

Her husband got her on to drugs, and the children were soon running wild. Within a couple of years, her second marriage broke down amid accusations of affairs and abuse.

She overdosed several times, but always called for help before it was too late. We were worried sick, but even when we managed to get her into rehab, she checked herself out after a week or two.

She couldn't cope with the children, and put all three boys - now aged six, five and four - into care. This time, social services wouldn't let her have them back.

They wanted to put them up for adoption, but our mother fought them at every turn, hiring lawyers and even offering to bring them up herself. My sister didn't seem bothered either way.

By now a mother myself, I found her attitude very hard to understand. It was difficult not to be judgmental.

I told myself she had an illness, and couldn't help herself. But it was hard not to get angry with her lack of concern, or the worry she caused our mother.

She admitted to still using drugs from time to time, but generally seemed to have her life on a much more even keel.

After her children had been in care for three years, she finally got them back, married for the third time to a man who seemed much more stable, and seemed to be turning her life around.

Then, once again, she went off the rails. Her husband left her, having caught her in bed with another man, although she claimed they were just "cuddling".

She attempted suicide, after which the boys were split up and put into permanent foster care, where they've been for the past four years.

I don't blame her for much of what's happened.

Shortly after she gave the children up for the last time, she was rediagnosed with bipolar disorder (in most respects it is what people mean by manic depression); although her new medication has helped to some extent, she'll always be more volatile and fragile than most.

But she doesn't make much effort to help herself either.

She only has one lung and was told smoking would kill her - yet she's smoked a pack a day since her late teens.

She knows that she should avoid alcohol because of her kidneys, but frequently binges, sometimes putting herself in hospital.

I don't know if she's still taking drugs - I hope not - but it's not something I like to ask.

Yet despite all this, I love her desperately. I'd do anything in my power to make her better. But if I were to give her a new kidney, I'd only be treating a symptom, not the cause.

Like George Best, who was given a new shot at life with a donated liver and wrecked that too with drink, my sister can't - or won't - change her behaviour.

I'm now a novelist and have three young children of my own to consider.

I'm the main breadwinner at home, and if I take risks with my health - and let's not forget that kidney donation is major surgery - I'm taking a chance with their future as well as mine. Who knows what might happen on the operating table, or later down the line?

I'm not a coward. If one of my children needed my kidney, I wouldn't hesitate. But my sister has made her own choices, and followed her own path. She's an adult.

My children have everything ahead of them. I'm not prepared to play roulette with my life and perhaps ruin theirs.

Perhaps, if things were different, it'd be a chance worth taking.

I'm not saying she doesn't deserve my help because I don't agree with her lifestyle - that's not the point. But with three young lives depending on me, I have to put them first.

The irony is, I know she'd be the first in line to give me a kidney if I ever needed it. She's impulsive and generous to a fault.

I just wish I could turn back the clock to the days when we thought a couple of scratches on our wrists was all it took to bind us together for ever.

She's my sister, and I love her dearly. But she's made her choices in life, and she has to live with the consequences. I just hope I never have to live with the consequences of my decision.

Tess's sister, Philippa Stimson, 36, is a mother of three who lives on benefits in a council house in Burgess Hill, West Sussex. She says:

When Tess told me recently that she wouldn't give me one of her kidneys if I needed it, I wasn't surprised.

She thinks deeply about everything before she makes a decision, which is the complete opposite to me.

I've gone through life leading with my heart, not my head. I suppose her way has worked out better, I mean she has a big house, lots of money, a loving husband and three beautiful kids.

I, on the other hand, am living off state benefits and at the end of each week I hardly have enough money left to buy a loaf of bread.

I admit I have made a lot of decisions that I deeply regret, mainly choosing the wrong men and not looking after myself properly.

However I, too, have three amazing children and while they are not living with me, I wouldn't change them for the world.

When I look back at our lives I am jealous of Tess's success. I'm not bitter but I do wonder how things went so right for her and so wrong for me. I think my health has been an overriding factor.

I have struggled with numerous infections and so I only have one lung and one kidney. Also in the past few years I've been diagnosed with manic depression - one of the main reasons my boys were put into foster care.

I know that Tess doesn't agree with the way I lead my life. I smoke 20 cigarettes a day and despite the drugs I take for my depression, I have a drink every now and again.

I shouldn't do either with my body in its weak state, but I don't think about anything in the long term anymore. I make every decision on the spur of the moment and take every day as it comes.

I know this attitude means I may need a donor sooner than I once thought, but I really don't blame Tess for making the choice not to donate her kidney. If I'm honest, I think I've always known she'd feel this way when it came to the crunch.

About 15 years ago, Tess was living in Cyprus and I went to visit her. I was pregnant with my second child at the time, and Tess was struggling to conceive.

We had a long chat about her options, one of which was surrogacy. I immediately offered to be her surrogate mother if it came to it.

She was very grateful but said she had to admit if I needed her to do something like that for me, she couldn't. I just saw it as an example of how different we were, it didn't mean I loved her any less.

And the same is true now. Tess knows if the boot was on the other foot and that I wouldn't hesitate to give her a kidney.

She is allowed her opinion as I am allowed mine. If and when I do need a donor, I will just have to hope I find one through the general donor list.

Every day I wear a necklace that Tess bought me for Christmas. It has three stones on it, one says "sisters", one says "friends" and the last says "for ever".

It is a symbol of our loving relationship and, one kidney or none, that is not going to change -at least I'm not going to change it. What Tess does is up to her.