This is more than likely going to be long and a bit complicated, but please bear with me
MIL and never had the best of relationships, DP was previously married (as was I) and we were good friends then, years later, we got together. But she is always going on about his ex wife, asking how she is, making a point of displaying her Christmas/Birthday cards in front of me. But when I fell pg with DD things seemed to get a bit better, we still have a slightly strained relationship, but it's much better than it was, or at least that's what I thought.
Now DP's cousin works at the same company I used to work at, so we have a few mutual friends, and while he was out with them (having had a few drinks) the conversation turned to mine and DP's engagement. He decided to tell these people that MiL had told his mum (DP's aunt) that DP was only marrying me because I had trapped him with DD, and that he deserved so much better. I'm still very close with one of these friends so they immediately told me.
I should add that the engagement wasn't a bit planned affair, DP just popped the question and caught me so off guard that I actually laughed at him and thought he was joking 
So I'm annoyed at MIL for saying these horrible things, DP's Aunt for telling her son, and him for telling my/our friends (who apparently were all quite shocked and a couple told him off for discussing something quite private so openly). I spoke to DP about it and he just shrugs it off with "you know what my mum is like" and thinks I should just ignore it.
But I can't seem to forget it, every time I've seen her since I can feel my blood boiling, because of our history I've always tried particularly hard to make her feel welcome and tolerate all the things that wind me up about her, like DD even at the age of 3 weeks having to fit in around her stupid dogs schedule, and when her dogs chase and pin down my cats I can't get upset because "they're just playing" and how she pays no interest in DD at all, but then gets upset that as DD gets older she has no interest in MIL.
I can't cut all contact with her, and I wouldn't want to as FIL is lovely (although I am grateful they live 8 hours away) but how on earth do I deal with this? I suggested to DP that when we next met up with MIL, Aunt and Cousin that I ask them all to explain exactly what happened, but he got really cross with me and said I was just stirring (which yes, ok I probably was with that idea)