Escape From memory by Margret Peterson Haddix?

the girl grows up with her mysterious single mother, who one day disappears. a woman shows up claiming to be her 'aunt memory,' is shocked when the girl has no idea what that means, and says her mother said to come with her. finds out that she is the daughter of heros of a culture that trains themselves to remember everything. during the war, soldiers used these people to remember code, etc., ripped their town apart, resulting in m immigration to california. the girl comes to realize that little of the original members of the race are left, and her 'aunt memory' is using her to get a special computer her parents made before they died. (oh and her 'mother' is actually her aunt) they use her 'mother' for leverage to obtain access to the vault with the computer, where the girl learns that the computer is actually in her head. with some hypnotism by her best friend who also gets captured somewhere along the way, she gains access to this information to out smart the villains so they don't kill anyone. fight scene, hostage situation, shootout, ensues. bad aunt commits suicide. the story ends with her discovery that her mother turned aunt is really her mother, who had stored her sister's memories in her brain via the brain computer technology out of guilt after her sister's death. she gives her mother a choice to which woman she wants to be.