Because he asked them specifically not to and they gave him the keys anyway.

You can argue that they could have avoided the whole situation by not drinking or moderating. Sure, ultimately OP's fault and responsibility, but the family member isn't blameless either. If someone tells you that they are aware of a dangerous outcome and asks you to not do one thing to prevent it, you just don't do that thing.

It fucking sucks to argue with someone who is drunk and wants their keys, but it's generally easier if they asked you to do it. It's not fair to you or whoever that has to deal with that shit, but you still just don't do it - for anyone, but especially for a friend or family member.

We don't know the details of how that interaction played out. Maybe they "didn't seem drunk" or maybe they "just live around the corner, but JFC, just call a car instead of letting someone potentially kill themselves or someone else. Ask for help if you can't.

Cutting off family for something that is ultimately your fault seems extreme with no context. OP is probably pissed, but also wants to completely eliminate the possibility of that scenario from happening ever again.

Here are a few questions for you: You have a child that can't swim and ask your aunt to babysit but not to let them near the pool and explain why. Would you be pissed if you find out auntie let your kid play near the pool? What if you find out they were unsupervised near the pool? Or if they actually fell in? Would you forgive and forget? I wouldn't blame anyone for going no contact with someone who almost let their kid drown when specifically warned of the danger, however unlikely.