Frog that's just unforgiveable on both sides, his and your mother. I remember from previous posts you saying that you are estranged from your parents. I could say they deserve each other, leave them to it, they're all as bad as each other, but I know it doesn't help.
I've mentioned before that I'm nc with ex's family, and he's nc with mine. Other members of my wider family were also friends with ex's family. Two of these, one related to my mum the other married into the family, played a large part in the sick saga that lead to mine and ex's break up. Like somebody said previously, the hatred I have for these two individuals is frightening.
Earlier this year my mum decided to get back in contact with her family member, despite knowing what they've done to me, my dad and my dc. It made me feel physically sick, I called her on it, and we didn't speak for a couple of weeks. I decided that I would not let these two people ruin anything else in my life. So I swallowed my pride, told my mum that I disagreed with her choice, wanted to hear nothing about the person, and that they were dead to me. She hasn't been able to stop herself mentioning that she's seen them. I think she hopes one day I (and my dad) will magically relent and see them again. This will never happen.
I'm writing about this as, although it's not as close to home as your situation, I get the wanting to scream, rant and feeling physically sick. In the midst of not speaking to my mum I bumped into a friend's dh in the supermarket. I was clearly upset but he went on and on. I just broke down in tears, it was mortifying, never said anything to him. I remember thinking nobody is ever going to make me feel so bad again. Hence the whole ice queen approach.
I don't know how you come to terms with such betrayal. That's the main thing I've had to deal with. I've decided that they're weaker than me, and only deserving of my contempt. I know I would never betray someone like they have done, so just file them away in the box labelled "done with". My mum and I are ok. I deal with it by thinking she's been a good mum, this is her one almighty slip up. I can't forgive it, so I just refuse to let it into my life. It's hard though. 