So... the one who actually committed the murders was the first child? Or was it really the second child? I remember inconsistencies in the story, stuff like her IQ changing and whatnot... but I think that was in childhood, and there was a whole history of her working and being involved with different men. If that was second child, and first child died before teen years, then how old was second child when doing all that stuff?
Also... if the girl who's been arrested was under-age all this time, that would explain a LOT about her erratic behaviour. A 20-25 year old psycho serial killer is one thing, but one who's 15? Possibly one who's been framed? The whole situation might make you actually go crazy and lose your mind.