Electronics, music, or gift cards to Target, Walmart, Best Buy, Barnes and Nobles, I know gift cards seem impersonable, but at 10 and 12 they can go get what they want!! But if you want to wrap something, make it electronics, CD's, video games.
With clothes I would give gift cards to Old Navy or JcPenneys or Kohls, being an aunt is hard, especially at that age, but I always found that if I gave them gift cards and then told them I would take them shopping and to a movie, that they were happy and appreciated being treated like a "grown up"
Goodluck,
also, my 10 year old daughter really likes cool blankets and pillows, and chemistry sets, that is what she asked for and at the last sleepover birthday party last week, the young girl was excited because she got 3 new soft fuzzy blankets for her room.