In Eclipse, the third book in the Twilight Saga, we find out through a flashback that Dr. Cullen has that Alice was in an institution. Alice does not remember much about her life pre-vampire-ism. We do know however, that her gift of "sight" is apparently something that she always had. While institutionalized, Alice was kept in the dark all of the time which explains why she can't remember a whole lot about her life.

Alice and Jasper are the last two to enter into the Cullen family (before Bella).