MIL drama!

I've posted before about not inviting my SIL at my toddler's B-day. She has a falling out with my husband and our child gets treated like stranger by her. MIL first said that she accepted it but now making unnecessary drama over it. She demands her siblings should sit together and talk it out. It seems like everytime there is an issue with my SIL she excuses her behavior but guilt trips others.

The issue is my SIL was pissed off after getting a decline for her house project and stopped talking. My husband has tried to be cordial and the "bigger person" by making small talk but she ignores him or doesn't look at him at all. So he is not at fault here therefore if the sister is actually interested to have a relationship with her brother, she should be willing to call him up herself. But my MIL is forcing things and it got my husband so upset and angry that he told her if she continues to hound him with this he's going to stop visiting. When my SIL didn't invite us to her birthday, her son's birthday, No one made a fuss about it and this isn't fair to us. I'm really holding myself back because my child is in the middle here that gets treated like she doesn't exist for my SIL.

I've noticed a shift in attitude from my MIL, she's short with me. Should I say something? Hold my tongue?