I understand your disapproval of the pot due to its illegal standing. There are certainly worse things she could be doing to manage her pain. Pharmaceuticals would be more addictive (IMO) and have tons of side effects as well.
If her use of pot isn't directly having an adverse effect on you or your family - then you should find a way to make peace with it. She's a good person, and you love her -- don't let this ruin your relationship.
I get really frustrated with my sister who won't give this WOE a try to manage her fibromyalgia - and honestly I would totally understand if she was using pot. From personal experience with the pain of fibro - I won't pass judgement on anyone who has found a way to cope.
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