I'm going through the same thing, though in my case, it's my dad defending my mom, and it's not SA, but physical and emotional abuse. I'm sticking to my guns, it's the only thing that works to at least keep her at bay. Unfortunately, that meant having to tell my mom to "go away" on Christmas, which made me feel like crap.

I know it was the only thing I could do to protect myself, though, and I would do it again if I had to.