Where are you located? If in the US, most counties have drug task forces with anonymous tip lines. If you don't want to say publicly, you can send me a PM and I'll try to find a resource. It's infuriating, because people who do this make it hard for legitimate pain patient to get the medication they need.
You would have to call her doctor, and discuss it with them. You can anonymously report the misuse/selling of the meds.
I wouldn't give the sellers name on the internet. It is unwise to give details such as names, cities, states, des names. Anything can be found on the internet and it would be dangerous to the family member, despite what they are doing.
The best option is to contact the prescribers office, or local police dept, and let them know this person is selling their meds. Both can direct them to get help.