NTA. Trust me when I say that I’m sorry it took ten years for your fiancé to show their true colors. You did 100% absolutely the right thing in this situation. Your fiancée sister was not going to get clean on her own. She was endangering her child and you honestly had no way of knowing if she was ODing. Even if she wasn’t that situation very much warranted a call to CPS.

Shame on your fiancé and their entire fucking family for treating you like that while enabling her. Do not feel guilty. I know ten years is a long time and you won’t be over it easily. No matter what happens you need to remember you did the right thing. I would definitely take care of yourself and go immediately NC with everyone involved including your fiancé. Also, fuck them saying they will sue you. There is nothing to sue you for. She knew you are coming by. She was high, possibly overdosing with heroin on the table which her daughter could have consumed as well. They all suck.