You would not be ratting anyone out. Abuse thrives in secret and makes us feel like we are bad if we tell. You can tell. You can raise the alarm, you can tell people that you are concerned. Suicidal threats are serious. And this is too much for you to have to carry on your shoulders.

Raise the alarm, tell people, call in help for yourself too.

If your sister won't call the hotline, you can call. I've done that myself in the past, called hotlines to get help for me as I deal with friends and family going through abusive and suicidal situations.

If you hear that he is making suicidal threats currently, you can call the police. They can do a welfare check. Let them figure out what to do next. You can set really clear boundaries and delegate what isn't your job to whoever does those jobs - in other words it isn't up to you to have to figure out when someone is seriously suicidal or not. You can call in community support and let the authorities do their work.

Set boundaries and let people have whatever consequences come their way for their behavior. Don't try to protect people in silence. You deserve better.