How do I deal with my mother and my older sister?

My 30 year old sister left her fiancee (a good guy but she said that he wasn't for her) about a year ago and my mother took it hard. She had begun to think of my sister's ex as a son and was really upset that my sister let him go and also felt the need to lie to my parents about letting him go. A year later and my sis is in a new relationship but mom won't stop going on and on about her ex! Now I get phone calls from them both and they each want me on her side!

Truthfully, I think they are both crazy. My sis shouldn't have lied but mom should have accepted her apology. My sis isn't as bad as my mom in this, she instigate and lies but is prepared to move on. Mom, though, drives me batty! She calls a lot and talks for hours at a time! I live two hours away and when I get family updates I don't want all of this negativity. My dad has washed his hands of them both and ignores them. My brothers have done the same. I would too except my boyfriend and I are going to visit for Christmas and I'd like it to not be uncomfortable. He knows all about this and is so supportive but it's not fair to him.

I just want the both of them to move on and I'm willing to help them get there but I don't know how. Any advice would be helpful!