My husband and I had a similar situation with his mother. She was sick for years with heart problems having both carotid arteries done and a quadruple bypass surgery. Then she fell and broke her hip, and everything really went downhill from there. She had to put her husband, who had dementia, in a nursing home. After all of the drama from caring for him was gone, she insisted on allowing a drug addict, adult relative live with her.
I think she had to have chaos in her life, and it seems it is the same with your sister. Although social workers stop by and see your sister, maybe you could ask that department to spend more time with her and talk out the issues she's experiencing with her husband. Although social workers aren't therapists, they are trained to look for patterns that result from abuse.
I know it probably sounds crazy to you, but people who live in abusive situations get used to it, and nothing else seems normal. It's a terrible cycle, but it can be broken, even in the late stage that your sister is in. If that doesn't work, you could petition the court to appoint a guardian for her. My husband and I had that done for one of his brothers who suffered terrible mental illness resulting from a serious head injury. The first guardian was a CPA, but a lawyer was appointed after it was determined that my brother in law had to be committed to a psychiatric nursing home.
Whether it's "therapy" from a social worker or a court appointed guardian, some of the pressure can be taken off of you. The fact that you have her doctor on your side counts for a lot. Basically, the courts will do whatever the doctor recommends. At least that's how it turned out in our case.
Good luck to you. I know you are in an awful situation.