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).
In Eclipse, it was brought up that she was in an institution and she has siblings, sisters I think who had grandchildren in Seattle. Alice wa kept a lot in the dark and that's why she can't remember it all.
In Twilight it came up that James knew something about what happened to Alice. James left clues and hinted at things in the video that he was going to show Edward once he killed Bella. Then I think in Eclipse it's brought back up aft that, around the time that Victoria's army was descending upon them, she was in a institution for visions or something before and that she was probably dead. But she had sisters and that had grandchildren so where in Seattle I think.