Doppelganger - Contains rape, non-con, kidnapping - WARNING: DARK FIC

What if Castle wasn't an only child? I recently saw a bones ep that presented a character who grew up as an only child only to find out he really had an older brother who was raised in a mental hospital. What if Richard's parents had a similar situation and Castle never knew he had an unstable brother. What if he was a twin? What if he escaped? DARK FIC - rape, kidnapping, non-con