Treating patients with a history of drug addiction for pain is not easy. The doctors have to weigh the importance of pain relief with the issue of potential drug abuse. The fact that your sister is aware of this issue is a good thing, because it opens a dialogue between her and her doctors.
There is hope for people in your sister's situation. There are medications that have lower addiction potential and experienced doctors can follow treatment plans that limit the risk of addiction.
The best thing for your sister to do is to have a discussion with her doctor and, if possible, a pain management specialist. By discussing the issues and her options, there is likely a solution for her so she doesn't have to continue living in pain.