Hope you can get it sorted. I have a similar situation every xmas, my husband's parents are divorced, and his dad took it really hard, my mum refuses to accept that we won't spend xmas with her, she has gone as far as to be threatening if we suggest not going, and my parent's, infact my family in general really do not like my husband's sister, so although they will allow us to invite his dad to xmas, they refuse to have his sister, and even if we do ask my FIL to come, he always refuses until the last minute, the last three xmases at least have been so upsetting for me, as we've either had to spend it apart to please both families, or spend weeks coaxing/blackmailing our parents into compromising.
It is annoying having to spend time with people you don't like, I guess sometimes (such as xmas, when I'm sure that you're mum would hate not to see you) it's got to be done, but something like mother's day you should be able to enjoy too, and it sounds like your SIL has over-stepped the mark a bit, especially with the comment that she 'felt like she had to' invite you.
Families are really not worth the hassle sometimes. Hope you manage to have a great day with your little girl.
