You are past the first hurdle, which is understanding that YOU cannot make your parents, or anyone else change. It's a sad reality that giving her an ultimatium may not do a thing. That is the reality, and I hope you are able to face that. Be prepared for your father to reach out in the future as he will have problems handling her as it progresses, which is normally does should they not seek recovery for themselves.
Keep coming back to SR. You are definitely not alone, and will get lots of good support here!