Thanks all for your input. I'm especially taking the "Don't call the ex" to heart. My husband was pushing me to call the ex, but I didn't think it was a good idea. If anything, I am trying to be much better about avoiding the rescuer/negotiator role in my family if it isn't necessary.
I don't think that this is a CPS situation, but I do notice that my nieces' antennae go up as soon as they detect any tension between me and my sister. My nieces and I are also VERY careful about showing any sort of affection with each other when she's around. For myself, I don't want to turn this into a competition for their affection. For my nieces, I suspect they don't want to encourage any jealousy/anger on her part.
My mom doesn't really enjoy my sister being there, and my sister totally picks up on this and feels even more rejected. I've told my mom that when my sister visits, she can't discuss my sister's pothead boyfriend and her current financial situation, because all that will do is just make my sister feel defensive about the choices she's made. I've reminded my sister that my mom has cancer present in her brain, and lord knows what is going to come out of her mouth (although her personality, so far, does not seem to be affected as much.) This is as much of a referee that I can be (I think my sister's angry about the fact that I don't referee as much anymore. And that I can't scrape up the extra energy to feel sorry for her. I've just got way too much fish to fry with my mom, and frankly I do wish she, as a forty-something woman, did take the opportunity to step up to the plate. But you've all heard that already.)
Again, thank you all.