How to talk to mother in law about her "problem"?

I have tolerated my soon to be mother in law for years. I've been with my fiance for 4 years, and known him and his family for 8 years. We have a small daughter together, in the past the woman has made no effort to hide her dislike for us dating even if she was good friends with my parents. My issue isnt that, it's her disregard for her drug use and my child's well being. I've only put my child in her care for no more than an hour twice in the 2 years of her life. His mother is a hypochondriac, always in pain always getting another "opinion" her daughter is now 11. The first time, my child was 6 months. She decided her phone call was more important and handed her then 9 yr. old our 6 month old. The 9 yr. old left her on an edge of the couch, my baby rolled off and landed on her head/neck area, thankfully unharmed. Second time was when she was 9 months, her phone was more important again, leaving her 9 yr. old in charge, my child climbed up 4 stairs, stood up and tumbled down, cracking her head on hardwood, thankfully uninjured. We told her not to put her child in charge, she was too drugged to care. She "hides" her use of drugs from her parents, my second realization was when her step mother saw her asleep again on our family trip last week and said she takes too many pills, keep in mind her parents rarely visit her and live in texas, she in missouri. She asked the next day to take our child for a week after the anual lake trip. we live 9 hours from them. My dad died of a drug overdose, so it is an issue close to my heart. I dislike her, but dont want her to miss a relationship with our child nor our child to realize her pill prob and alcohol prob. Any advice on how to tell her to get clean in a nice way? my fiance says it isn't our business, I believe it's mine if she's taking our child in any cirstances in the future.