I agree with everything you say op, I really do.
But I have four dc, all late teens and twenties, and managing disagreements between them now they are adults often makes me uncomfortable.
Sometimes I'm told something and asked 'not to tell the others' and I don't, but I often feel like I'm lying or deceiving them in some way. I would much rather they didn't tell me in the first place, so I'm not in that position so maybe that's where your mum is coming from too?
It's sad that you think your mum values your sister's feelings above yours. I'm sure it's not the case. If you feel sidelined it is probably because you appear reliable and sorted, needing less support.
I can't imagine watching my child destroy themselves through drugs, living through their overdoses, being told they won't survive the night.
You are right. Your mum should respect your wishes to be nc with your sister, but I know in her heart she will be desperate for you to reconcile. Your sister's threat to you was awful but it was also several years ago and when she was unwell. You are under no obligation to forgive or forget but your mum will want it so much.
That's all I'm saying - she was indiscreet to tell your sister about the hospital appointments, and you're understandably angry, but she probably feels it was such a minor thing and now here she is being on the receiving end of yet more grief.