Yeah is kinda what I was thinking too.  Maybe daddy even is mean to dee because he looks too much like momma did or some such bullshit.  And yeah dee so would resent dum for being daddy's girl and getting things her way.  At same time since she is his twin he would feel like she was his possession so if anyone tried to mess with her he would go apeshit.  However we still need to figure out how and why they end up at the asylum.