<human>: Can you condense the text into a few key points?

My dad died when I was two in a car accident.  My mom took it hard, became addicted to painkillers, and then meth, and was the worst mother possible to me without abusing me.  She just wasn't there.  She left me alone for a week when I was fourteen, and I was so scared I couldn't tell anyone.  She came back, I yelled at her and cried, and somehow everything got back to "normal."

When I was 17 she went to rehab, and has been clean ever since.  She married some super-Christian guy, had another son, and seems happy.

I've never forgiven her.  I felt like if I didn't at least outwardly forgive her I would be actively sabotaging her attempt to get better, so I went along with it.

Her husband is obnoxious to me, he's always talking about religion, while I just don't care and won't care until my deathbed.  I don't feel anything for my half-brother, and refuse to baby-sit or do anything with him, I've just told them it's because I'm nervous around babies.

I just decided that I really don't want them in my life, my fiancee just told me she's pregnant (we're happy) and I don't want them around my child.  I feel like I individually could put up with them, but I guess it just makes me mad that she could be such a shit mother, and still call herself a grandmother to my kid.

What's the best way of removing them from my life?  I don't even want to be particularly cruel, I want them to know because I know how it feels to be left without a word, without knowing anything.
<bot>: Need to cut off my mom and her family because I don't want them around my kid, can't just disappear.  How do I tell them this?