Escape from Memory — Margaret Peterson Haddix

While playing around with hypnotism at a party, Kira remembers fleeing a war-torn country with her mother, speaking a language she can't identify. A few days later her mother disappears, and a woman who calls herself Kira's aunt Memory takes Kira to Crythe, a country that doesn't officially exist, in order to rescue her -- or so she says.

Kira soon learns that Aunt Memory is not what she seems, and that Kira and her mother are both in terrible danger. There are memories locked in Kira's mind that could get her and her mother killed. But those memories are the only things that might save them....