What would your say?

My mother in law lives in an old country style two story house. And watches our two yr old sometimes at her house. We have told her over and over to put up the baby gate on the very steep very sharp stairs. She keeps purposely not doing it and her excuse is she watches her so well that she never goes to the stairs. Just last week we went for a visit and I was watching the baby play and looked away for just a moment and mommy senses went off. My little one was 4 steps up!!!!!  Nothing happened but it could have. Even after that event  his mom still isn't putting the gate up 😡. Hubby has asked his dad to talk to her but I have very little hope that will work. Has anyone gone through this? Advice? Suggestions? She wants to watch her over night so she doesn't have to drive out to our house cause she is my nanny of sorts during the day cause I work from home. Our stairs have been and are gated. I'm afraid something will happen and if you could see how steep these stairs are you would understand my fear. And at the moment we limit her time out there mostly for this reason  Forgot to add that we are attempting another sitter, but my child is painfully shy and has stranger anxiety.