Drug addict MIL

Hi y’all.

I need a bit of advice. So my mother in law has been an on again, off again drug user (meth) for 20+ years. She’s a nice lady most of the time and my fiancé obviously loves her. However- she’s a liar and she steals. I never know when she’s clean and I have to make sure she doesn’t have take our stuff when she comes to her house.

She recently came over for a few days and she helped around the house a lot and was babysitting my fiancé’s son. He trusts her because she’s his mom, but I just don’t.

Anyways, I haven’t vocalized this to my fiancé yet because 1) I don’t have a say if she babysits HIS son and 2) our baby isn’t going to be here for 6 weeks. I know it’s going to one day come up where he wants her to babysit our daughter or let her be alone with the baby. But, I refuse to let that happen unless she can pass a drug test. I can’t have someone unstable alone with my baby.

Is that unreasonable? How can I approach it with my fiancé in a way that he might understand more? I don’t want to hurt his feelings or start a war- but that’s a hard NO for me.