I need to write it in such a way that makes them unaware of each other's existance. The male twin runs away at 12, but by that age if he were told of/knew of a sister I'm sure he wouldn't just forget about it; he would seek her out and refuse to leave her. But if they were split up when they were very young (I'm thinking before ages 5-6, ideally as infants) it's possible they might not remember each other.
EDIT: While I'm at it, is 90-95 pounds obscenely thin for a teenaged boy about 5' 11"? He's naturally very thin, but the starving/purging/obsessive exercise makes him drop off the weight at an alarming rate. Around what weight would you say he'd start looking emaciated and disturbing? I can find plenty of symptoms and accounts about anorexia, even if it's mostly women, I'm just having trouble pinpointing a specific weight, and since most of his narratives revolve around him obsessing about it, it needs to be specific.