Double faced mil just vent

My mil will bad mouthing my SIL in front of me,and when I’m not there mil will bad mouthing me in SIL presence.

So no matter what both us did for her it will never be enough.

SIL is person that will tell you everything but I’m trying not to argue with mil because if I said something she would add more stuff on what I said so I just give up.

And what is bother me why she need to be double faced.

SIL is wife from my husbands brother