I think, from what you said, that you know some of the police officers personally?
Perhaps you can call up someone you know, and tell them about the situation - that your parents are harboring a fugitive out of some misplaced sense of family loyalty, and that they want you there for "Christmess", and that you don't want your children to have to suffer a police raid.
Thinking aloud - on the other hand, I don't see that the police would agree not to raid just in order to spare your children danger, so probably the only help would be in having your brother picked up first (which I'm 100% in support of).
I wonder if lying would work - just tell your mother that you'll be there, and then don't go and don't answer the phone. She'd be annoyed afterwards, but hey, maybe she'll forget after the police raid happens. She seems not to connected with reality, and I'd be worried about exposing my children to her anyways.
The only person who can make you into a door mat is yourself. I rather get the impression that you need to say NO to your mother more frequently (NO, I'm not going; NO, you're not looking after the children; NO, you're not making good choices). She doesn't seem to support YOUR choices, the same way that your brothers are supported.
BTW, I remember a situation in this province a few years ago - the police had staked out a house for some purpose - drug raid? escaped convict? - and then when they raided the house, shot the dog (who was not pleased with the raid). Unfortunately, they had missed that a children's birthday party was being held at the time. This made the news as being rather traumatic for the children.