Background: My mum hates my MIL and has done since she first met her. I can understand why she might dislike her - their personalities clash dreadfully (they're quite similar in many ways!) and I find my MIL very irritating myself sometimes, but her heart's in the right place and she's always been good to me. The absolute hatred is irrational and unfair and puts me (and DH) in a very awkward position. I wish she didn't feel that way and, failing that, I wish that she would rise above it and find it in herself to be polite and fake in the English way! I've given up on that, though, so I just make sure they never see each other.
MIL looks after my DD one day a week at our house, which I know my mum hates, but, as I've said to her, it's nothing to do with her. Mum sees DD much less often as they have lots of livestock and it's very hard for them to get away, but she adores DD.
Anyway - the point of this post - yesterday MIL was here, and mum phoned to ask after DD. MIL started speaking to DD quite loudly (probably so my mum knew she was there - she's not entirely guileless), at which point my mum started saying: 'Oh God she's there, I can hear that awful voice, oh why won't she just shut up, etc.' As I was in the room with MIL and wasn't sure whether she could hear my mum's voice, I was mortified and didn't know what to do. I cut the conversation short and tried to act normally, but I was absolutely seething with my mum. I don't think MIL heard anything but I don't know for sure.
What I want to do is confront my mum for being childish and rude and for putting me in such a difficult position, and tell her I'm sick of her attitude and she needs to get over it. I know from experience that this won't go down well. What would you do? AIBU to expect my mum to act like an adult?