OH wow,,

Was I in your situation. I opened up my mouth and said how I felt and I ended up looking (and feeling) like the bad guy and the damage that was done between my sisters and I.. well, althought not all my fault. I guess if I had not said a word and let them continue on bashing my mom, it all would have been blown over and none the worse. But I didn't do that, I told them how I felt about it (and you know, I don't regret it, really, I mean, I would probably regret more if I hadn't of stood up for my mother).

This is a hard one. If you are very close to your sister, I would say go ahead. But if you are not very close to her, I would say to really think carefully on this one.
It may draw a wedge bigger than can be taken away..