Hi, Matt--

Your sister's mental illness is not an excuse for her abuse. Unfortunately, though, parents can focus more on the "sick" kid and not much on the "healthy" kid, even if the latter has serious problems. That's what happened to my brother and I. He was "sick" and I was "healthy", even though his issue was resolved with surgery. He managed to mooch off my parents for years and I was kicked out at 18.

You do need therapy for the abuse. Maybe you ask for it under the guise of "helping" your sister. If I were your parents I would also consider family therapy.