Annoying mother

Hiya just came here to have a little rant and ask if I’m being stupid or reasonable. My mum loves her grandson but takes over all the time. She gets very angry when I don’t let her baby sit him but last time she left my 8 month old son with my autistic 14 yo sister alone (after I made it so clear I didn’t want her doing this) My sister has the mind of an 8 year old, she dropped him and he smashed his head into a table, my mum called me like it wasn’t a big deal when he had a huge lump on his head, she said he didn’t cry and I know damn well he did. (He’s okay now thank god). So I let my partners parents babysit him now when we need but she says she’s gonna tell the rest of my family that I’m letting them down and I don’t care about them. Today my baby’s car seat broke, me and my mum had plans but I let her know that I couldn’t come bc I had to go get a new one for my baby’s safety and she absolutely blew up at me. Her birthday is Sunday and she’s now “punishing me” by not letting us see her on her birthday but then pretending it’s bc idc about her, when she’s actually spending the day with someone else’s family. ( she has a bf but is Inlove with this other man, she’s going with this other man’s family) so frustrating!! That’s only scratching the surface she ruined my pregnancy and treated me like a maid not even 2 weeks after my c section