Given that The Breeders release albums less often than February 29 appears on your kitchen calendar (this month's Mountain Battles makes just four Breeders albums in the last 18 years), guitarist Kelley Deal might be better known for playing colleague and caretaker to her twin sister, Pixies bassist and Breeders leader Kim, than for her musical abilities. After all, that's the role she played in loudQUIETloud, the stellar documentary of the Pixies' successful 2004 reunion tour.

See, Kim, who went through drug and alcohol rehab in 2002 (Kelley, as older sister by 11 minutes, served as predecessor following a 1995 heroin bust), agreed to the Pixies restoration on three conditions: There would be no alcohol in the band's dressing rooms, she would have a separate SUV to travel in, and her sister Kelley would be able to come along for the ride.

"You know, Joe [Santiago] and David [Lovering] and Charles [Thompson, aka Black Francis, aka Frank Black], although each and every one of them is a really cool, interesting, funny person, they're guys," Kelley says of the three Pixies not named Deal. "And, I don't know if you know this, but guys aren't good at going shopping, having coffee and talking. Especially those three guys. There are some [guys] that have more female in them than others. Those three have no female in them at all, so my role on that [tour]...honestly, I was a companion. That was my 'job.' You know, just hanging out, being a sister."

Info Kim Deal The Breeders perform Tuesday, May 6, at House of Blues.

But with the Breeders—Kim's band since its 1988 inception; Kelley joined after the recording of 1990's Pod—her responsibilities become a little more defined.

"If Kim is the quarterback," Kelley says, taking a dip into the pool of Midwestern sports analogies, "I'm the center and the coach. Can I be both?"

Absolutely.

Such is the honest, open and unabashed charm of the Deal sisters.

And the job of the offensive lineman is a noble one, the human equivalent of a well-trained watchdog. Faceguarded in relative anonymity, those hefty protectors of the so-called skill positions are loyal to a fault. Underappreciated and underrated all the while.

Evidently, this strikes a nerve. Because, suddenly, caretaker Kelley changes her mind.

"One of the other guys [ostensibly either Breeders drummer Jose Medeles or bassist Mando Lopez] can have that then," she says. "I don't want to be underrated."

Perhaps because they are sisters, ever-present for one another, the two need to be each other's rocks—not on the rock 'n' roll road but, more likely, in the Deals' hometown of Dayton, Ohio.

"There is a symbiotic relationship," Kelley says of her interdependence with Kim, "where I take care of her, she takes care of me, I take care of her, she takes care of me. It depends, on that day, who needs taking care of.

"When I was doing the Kelley Deal 6000," she says of her 1996, post-rehab band, "I had just gotten sober, so it was a pretty precarious feeling. And this girl came up to me and she had a couple bags of heroin—powder heroin—in her hand, and she started to give it to me and I just turned away and fled. But you know what? I wasn't tempted. It kind of freaked me out, but at no point was I ever tempted to take it.

"However, let me tell you, when I go to what I like to call 'civilians' homes,' and I go to their bathroom and I open their medicine cabinet—because that's what I do—and I see Vicodin there, I tell you, over the years I have popped in there and I have taken Vicodin out of people's medicine chests," Kelley continues. "And I find that is more of a slippery slope. In my parents' house, in my brother's house, in my friend's house, in this stranger-that-I-don't-know's house. I find that way more precarious than being handed a bag of smack at a concert.

"Isn't that strange? Because somehow it's not real. It's medicine. So if I were going to relapse, it's not going to be on heroin on the road, it's going to be in your medicine cabinet."

Kelley pauses, either to let the effect of her words hit or to allow them to boomerang back onto herself. But then she laughs.

"That's so depressing," she says, between chuckles. "When I talk about shit like that, it's like, 'What is wrong with me?'"

Yes, a conversation with one of the Deal sisters, like the music of The Breeders, is wild, wacky, waggish and more than a touch whimsical. Mountain Battles offers an especially idiosyncratic, intoxicating mix of melodic innocence: single-finger guitar parts (another reason Kelley may be better known as a caretaker) over schoolyard rhythms with matching messages of musical guilelessness. Kelley sings a song in Spanish (though she doesn't speak Spanish). Kim sings a song in German (though she doesn't speak German). "Istanbul" follows phrasing presumptively purloined from a jump-rope session ("Where ya going?/To the city/Where ya going?/To the city/Where ya going?/Is-tan-bul!"), and Battles begins with "Overglazed," an infectious calling card in which the words "I can feel it" represent the lyrics in their entirety.

And yet the sisters' quirkiness resists the simple constraints of self-deprecating confession (especially if ragging on your identical twin doesn't qualify as self-deprecating). Both are legendary cigarette smokers, but Kelley quit more than two years ago (and will gladly recycle the pop culture trash when mentioning her entrapment "in a shame spiral;" you see, she's still chewing nicotine gum), while Kim's cessation can only be counted in months.

"She's having a really hard time with quitting the cigarettes," Kelley says. "For some reason, her quitting the cigarettes has made her re-evaluate everything else she's quit. Like she's not had a problem staying sober at all. At all.

"But I tell you, since she's quit smoking, she makes these jokes, and I know they're jokes, but how much are they jokes, really? 'I would love to have a beer and a cigarette right now.' And she didn't say that before. I think the quitting smoking has been harder than quitting drugs and alcohol for her."

OK, so if Kelley Deal's role in the Pixies was "companion," what exactly is her role in The Breeders?

When the Pixies toured and Kelley was along for the ride, Kim was still working on Breeders songs. In fact, "Walk It Off" was first recorded in Kim and Kelley's motor home (a scene captured in loudQUIETloud). But though Kelley actively assisted as advisor and advocate during the writing of Battles, she receives no songwriting credits.

"A lot of times, just because it's not on the album itself, [it] doesn't mean that I won't be getting any royalties from it," Kelley says with a laugh. "One thing that I've discovered is when I'm with Kim—and I figured this out and I'm OK with it because I analyzed it—it's like, 'What's going on? Why am I not writing songs for the record?' And something that I discovered for me is when I'm doing Breeders stuff with Kim, that's not my job. My job is to be encouraging, to help, to collaborate, to come up with parts, to come up with arrangements—things like that. But not to bring the lyrics to it. That's not my job."

Does that mean that The Breeders are, in fact, her sister's band?

"No," Kelley says and laughs again. "It's Kim's and my band, but I let her think it's her band. And she probably lets me think it's our band. It's a total mindfuck."