You can see how this arose though.
The op talking about her IVF, Jenny saying something like I can’t wait to be a granny, and the op responding with well you won’t be, you’ll just be Jenny. And whatever else she said.
Because whatever was said, it’s clearly bad, for them to take this route.it was not done with kindness. The op calls it a “spat” which means there was an argument.
The fact she says the father has been trying to mediate, indicates it is an argument that went on, not something where the op said, sorry, I wasn’t trying to hurt you immediately.
She also says she has “ maybe taken some subtle shots at her” I think we can all guess, there is no maybe about it, and none of them were subtle. An under statement, like the “spat”.
Her AIBU is whatever goes on with her and the step mum, read, whatever she does to her, shouldn’t impact the relationship with the dad and half siblings, which shows that that’s what’s been happening for the last seventeen years. Whatever she did, it didn’t impact. So she felt free to bully or belittle her.
Most decent parents would eventually stand up and say that’s enough now. You can’t just treat people badly and expect everyone else to keep saying nothing and forgive you.
One thing for them to do it when you’re a child and struggling, it’s a whole other ballgame when you’re w thirty year old woman.