crack cocaine adiction, rehab, relapse, now using pain medicines?

Hello, My husband’s sister was in rehab for crack cocaine aprox 3 years ago. She was clean for a while, abused pain meds for a time, then went back to crack.This was before I married her brother. She is 50 years old. She asked if she could move in if she left her still crack adicted husband. I feel sure she has not used crack since moving in-aprox 5 mos ago. I do not know much about addiction, what is OK & what is not. My problem? Due to an auto accident 30 yrs ago my husband must use strong prescribed pain med. His Sis started asking for & taking his pain meds,1 or 2 every couple of days. When I found out, I said I didn’t think this was OK, she said “at least I am not using crack” While my husband agrees with me he is swayed by her implied threat that if he does not “share” she may go back to crack. Also he feels that she is an adult & must make her own decisions. I think he wrong to provide her with pain meds.
Am I wrong, Is using pain meds OK if it keeps her off crack?
Thanks

More Meth Drug Addiction Info: