Aren't you using you're SIL for daycare now? Is MIL involved in this? Or are you talking about just in general leaving the kids with her now and then? Maybe you just need to have a sit down - face to face with her about it. Tell her what you're concerns are and see what she says. It doesn't need to be anything confrontational, just a "lets have a talk" thing.
As for her wanting to keep it from you - well, she already knows how you feel about cuz I know you've talked before. I wonder if she is feeling uncomfortable about it herself and doesn't want to have to worry about what anyone else is thinking.
People will do all sorts of things to block pain. Before I found this WOE - I strongly considered getting a medical card to use pot. And I've never done any kind of illegal drugs - I didn't even drink alcohol until I was 40! and now I wish I had never started that! The pain of fibromyalgia is relentless. Its like having a migraine in your entire body. I don't blame her for using what works to dull the pain - but since its illegal she shouldn't be doing it. Better to take a legal narcotic in my opinion.
I totally understand your frustration with her unwillingness to try dietary changes. My sister still hasn't even though she see's what a difference it has made in my life. She says, "one of these days I'll get it together and give it a good shot, I'm just not ready" Thats just the same as saying, "I like being this sick and I'm not ready to feel better" Makes me so angry!
Bottom line - you can't control what she does or who she chooses to hide it from. But you can keep your children from being in her care unless she can promise not to be smoking it before or during the time she is with them.
1. Love ME no matter what noises are screaming at me, or who is trying to tear me down.
2. Eat to heal
3. Move to live
4. Embrace today
5. Live with intention
6. Respect my body
7. Cultivate joy
8. Find my passion
9. Meditate on peace in my soul