I'm going to try to help my sisters boyfriend but realize he can't be helped and back out
Why don't you think you can help him?
The common factor in this conversation is people who I help but when the situation gets tough I bail.
It's great that you want to help him out. If you told him you are going to help, you should try to do your best instead of bailing on him midway.
I can't stand seeing someone I know going completely crazy and dangerous.
That sounds complicated and overwhelming. It's good to be there for people especially during the challenging times. Finding a professional that could help would be a good idea too if the situation is over your head.