Help?

Okay friends, I need some help. Moving is hard. I'm very stressed and confused right now. For years, I've been listening to SIL say mean things about a person that I care about, MIL. I usually just ignore it, but now it's all the time. I'm being told about things that happened before I was even around. She is really dwelling on this stuff and I just don't want to be dragged down. Now SIL is telling me to look for certain behaviors, but I don't want to see them. Maybe I'm just over-thinking things here, but it feels like SIL is trying to sabotage my relationship with MIL. You know that saying "misery loves company." There have even been times when SIL lied about working late, just so that she wouldn't have to see MIL. I want them to have a better relationship, but I don't want to be in the middle. It makes me uncomfortable. When BIL comes into the room she stops talking, she obviously doesn't tell him as much. I know that she lies to BIL about it. I just don't want to see anyone get hurt. I don't think said person has many friends, maybe she just feels that talking about it helps. I've been told some things that I really don't want to know. I don't know how reliable this source is because she has lied to me before, I guess anything is possible. I've had a really good relationship with MIL since the beginning, but now I'm being told that MIL is saying mean things about me when I'm not around. It really hurts my feelings and shocks me. I just wish I knew what to do. PM with any thoughts on how to deal with this situation. Any advice is appreciated. Thanks!