I wanted to come back and give you all an update, but, you guys, I'm SO DISAPPOINTED I'm going to lose out on my TK Father of the Year award to psychodad a few threads below. Totally crushed.
My sister is now officially moved in with us. We had her stay over the weekend, as someone had suggested, and it worked out perfectly, just as we knew it would. She said she wanted to move in ASAP, but didn't want to deal with our mother. I called my mom and told her what the deal was, and got off the phone before she could pitch a fit. I (stupidly) had my sister tag along when I went to pick up the rest of her stuff, thinking it would make life easier to know exactly what I was supposed to pack.
We went to the house when my parents weren't home to get packed, but my mother came home before we left. I had my sister get in the car, but not before mom was able to scream some really terrible sh!t, and basically threaten to sue her and have the child taken away. I've told my sister that absolutely cannot happen, there's nothing our mother can do, but she's still pretty rattled. At least she's out!