How can I help my mother when she doesn't want to get better?

I have a family situation that I am extremely concerned about and don't know what to do. I could really use some advice. My parents are in their early 80's and took my older sister (age 47) into their home 5 months ago to care for her. My sister has Lupus, but 1 year ago she really went down hill. She took to her bed and stopped eating. She refused to go to the doctor. Her husband finally took her to the ER after she fell down the stairs and gashed her head. At the hospital they found stage 4 bedsores on her sacrum. One was the size of a silver dollar and down to the bone. She developed osteomylitis. She weighed 100 lbs. She was put in a nursing home and after six months used up all the insurance benefits (it was a very nice one in California.) Her husband said he was unable to care for her so my parents took her in (they live in Oregon.) She starting receiving excellent medical care, was put on Lexapro and seemed to be getting better. However, any time anyone would mention "wow, you are doing great." or tell her she was getting better and start expecting her to do more things, she would suddenly take a turn for the worse. Examples: She was unplugging her wound vac and her sores would get infected again...she developed pancreatitis and diverticulitis but would refuse to eat right and then end up back in the hospital...she won't do PT because it "hurts too much" and was still using a walker after 5 months. At Thanksgiving when we were all together and the walker started getting too easy and my brother mentioned that perhaps it was time to graduate to a cane, she had a "fall" and is now back in a wheelchair. She won't even use her arms to move the chair, she insists my parents push her! She uses a bedside commode. If she had cooperated with PT she should be getting herself to the bathroom on her own by now. A few weeks ago the hospital discharged her to transitional care in a nursing home because she needed IV antibiotics every 6 hrs for yet another infection. She called my parents at 6 a.m because "she couldn't stand it and wasn't getting any sleep." They immediately went to pick her up and take her home. My mother would get up at night to administer her antibiotics. My sister "was too squeemish to twist on the cap". When i visit my parents I see how exhausted they are. I've offered my help whenever I'm there and tried to tell them they need in home nursing care or she needs to be in a nursing home. I thought of moving her in to my home, but I have 3 kids, a full time job and a very small house. And My husband refuses to even be around her. Plus, She would have to establish with different doctors if she was here. It's like she WANTS to be an invalid. Her wounds are healed, her weight is normal her labs are good and she's on the right meds. Why does she keep making herself sick? Why does she refuse to do anything to get better? My parents refuse any help! They think they can do it all themselves. "We're family - it's our job!" So, they too are part of the whole sick situation! Please, have any of you ever encountered a situation like this? What can I do?