My 12 y/o dd is 5'2" and weighs about 85lbs, but she has a rib cage more in the 30"+ range. Your family must be really tiny in the rib area! My dd does wear the Aeropostale camis and I've never found them to fall apart quickly. They seem to be fairly durable, but she does wear xs in their camis and, while she is a 00 in juniors pants, she sounds bigger in the rib area than your dd.
I thought that Justice made some plain camis in that dd10, who is tiny (maybe 54 lbs.), has some older Justice camis of her sister's in size 8s. Maybe they have moved away from plain plus I imagine that those little sizes might be too short for your dd's height. I guess that I'm not much help at all -- lol!
Have you tried any of the other juniors stores like American Eagle or Hollister (although my dds hate that store!)?