OP, I am really sorry that you have this to deal with, and all around Christmas.
You spoke up, which I think was understandable and reasonable in the conversation context. But what is not reasonable, I'm afraid, to my mind, is anything your mother is saying or doing on this front.
You now regret saying anything and are scared and worried you won't be able to have a nice Christmas with family because you spoke up to your mum. That's also not good.
She is your mother, but she sounds rather tricky to deal with, especially as her child (this is an understatement, I don't want to inflame your feelings at all - best just calmly to look at what's happening).
As for the inheritance, I think that what your father gives you is his business, nothing to do with your mother, they are not together. She got a lot more money from the divorce than he did, as well, as I understand. It sounds like she feels the money in the marriage was more from her than from him, so she feels she had more right to it and has more right now.
In my mind, she should divide her estate equally between her three children. Presumably the half-siblings will inherit from their father also? Or is the inheritance she's giving them the shared inheritance from her and their father?
I'm sorry, OP. I'm afraid I think you should apologise in a simple way to her, and hope to have a happy Christmas. Apologise really only because you don't need more upset and to be cut off from the family you want to be with. And also just remember that sadly she's not reasonable, so you are a bit on a hiding to nothing trying to talk to her. x