Welocme!
You sound like a wonderful sister!
It sounds like it is time for a serious intervention with a professional.
It is time for her to agree to treatment or your bil needs to divorce her and keep her away from the daughter. What a sad situation. But it also sounds like your sister is crying out for help and needs a little push.
Keep reading and attend Al-Anon and Nar-Anon. I would also encourage your bil to do the same.
Best of luck!