As others have said, you need to do whatever is necessary to keep yourself and your children safe. No matter what your mother thinks, what was done to you DID happen and WAS NOT your fault. Your mother is probably scared, maybe she wasn't believed when she told someone what had happened to her - or maybe she was told it was her fault. Or something. But it doesn't excuse her behaviour. And you do not need such toxic people in your life, or in your childrens' lives.
I imagine the thought of losing your relationship with your mother is painful, just as much as the thought of "having" to have contact with your half-brother is painful (and angry-making, and sickening, and a million other things). But is maintaining the relationship worth the cost?
For me, it wouldn't be.
My mother has cut off contact with her mother over similar issues - not that my grandmother doesn't believe what went on, but she never did anything to stop it (my grandfather abusing my mother, her sister, her cousin, possibly her brother,my grandmother's sister, my sister, possibly me, possibly other children on the other side of the family, and who knows how many others) even though she has to have been aware of what was going on.
A few years ago my grandfather tried to make contact with me (through my family, not directly). The very thought of it just made me feel sick - and we don't even know if he abused me, there are only hints that he might have, not definite memories etc like others have. But while I still see my grandmother, there is NO WAY I will ever see my grandfather again, and NO WAY he will ever, EVER meet any of my children.
*hugs* to you. It's an awful thing to have to deal with.