The good news is that it's normal
If it's anything like my marriage, he really doesn't care, and everytime she insults you it drives him away from her and closer to you.
In all honesty though, everytime she insults you, she insults him and his decision making process.
I'd just shrug it off as best you can and try not to retaliate. Minimize contract if you need too.
But I've got mommy issues so take my advice with a grain of salt.