I think your husband should stand up for you (don't get me wrong, I'm not saying that he hasn't or doesn't). You're his wife and she's his mother and I think he should say something to her in a nice but forceful way. Mother's will always forgive their children but may have a harder time with the "In-Law" and she may take it better coming from him. That's how my husband and I deal with "family" issues. If it's my side I handle it, if it's his side, he handles it. Thank God we don't really have any issues on either side.
I remember when we were first married and I was pregnant with our youngest (I have 2 older boys from my first marriage). Before our son was born he told his mom that he expects her to always treat our 2 older boys the same as our youngest and never show favoritism or she won't see any of them because as far as he's concerned they're just as much his children as the new baby. We've never had a problem (we had reason to think that this could be a problem). Good luck to you
