I really disliked this one.
It seemed like I was supposed to relate to the protagonist somehow, but I just really didn't. She was a psychopath through and through. Her sister wasn't likable either, but she was the lesser of two evils.
So... if you don't want to be a mommy, you deserve to die? And, while I don't like the sister, inflicting her with cancer if she doesn't see things your way is evil, cold, and evil.
It sounds like the nanos will give immortality or something near it, at least protection from aging. The world is overpopulated as it is, so if you suddenly provide a product that significantly increases lifespan then there'd better be a way to reduce birthrates simultaneously or you're just multiplying the problem. With all of our medical science these days, the birth rate has also gone down a lot from what it was a few centuries ago--this makes total sense because these days the survival rate of the kids is very very high compared to those days. So if most of the population gains extreme longevity and at the same time makes themselves sterile, I don't really see where the problem is. The protagonist wants to have kids, and no one is trying to stop her, so what's the problem? She can have all the babies she wants, and can continue with her own scientific experiments at home, perhaps even ones which undermine her sister's work. Instead of stewing over it, she could do something with her talent. But if infecting her sister with cancer is really the best solution she can come up with, then she's not much of a scientist or much of a human being. I shudder to think what her twisted mind will do to her children.
And, with this one combined with Nano Comes to Clifford Falls, it's hard not to perceive a continued message telling me how important it is to have kids. Since I don't really have any desire to have kids, this bugs me. I realize the characters aren't the author, and this is a very small sample size, but it's not much of a jump to conclusion since these two stories ran only 6 weeks or so apart. This is probably multiplied by the fact that I heard them 2 days apart.
That said it was well-written. I hope to hear more Nancy Kress, but hopefully with a broader range of themes.