Thanks everyone for your feedback re: my mom. I thought and thought about it and then called my brother and told him how I felt about everything. The way Marie treats my mom, how I wanted to move her down her but didn't see it being a reasonable thing to do, etc.
He told me for one thing that he is thinking of moving her to Boston. He has plenty of room in his house and his partner is on board with it. Plus she'd have the added benefit of having Boston doctors. But that can't happen until he's done with his doctorate program in about 7 months - 1 year.
In the meantime he and my other brother at the Cape are going to bring her to their houses each once a month to give her something to do on the weekends (when her depression is worst) and to give my sister a break.
And I called my sister and told her if she needs someone to vent to or just talk to (seeing as she doesn't go to therapy and very clearly should) I can be that sounding board for her.
I feel a huge sense of relief. I'm bringing my mom down in January to visit for a week or so.