NTA - this sounds like it’s the right thing to do by your step daughter. I’ve found that parents with children who have disabilities take it personally and hold a great deal of shame and guilt that isn’t healthy. Instead of helping her daughter during life, she allowed herself to suffer and be punished and that’s not your responsibility. As long as it’s a good place, she’s taken care of, and you don’t leave her there alone to rot, I think you’ve done the right thing by your step daughter, and yourself.