I know. I work with persistent mental illness, those who will never be cured. They cycle and are in and out of crisis centers often. And, families can't do much, except not write them off, as so many do. We are sort of baby sitters for them. They come in for breakfast, and stay all day. They have access to computers, arts & crafts, music, TV, along with other activities, as well as group sessions, which are voluntary. Maybe your sil would find some comfort in a place like this, as well as you and the family. We aren't a cure, but we are hope. And yes, it is sad, very sad.
It sounds like you are doing as much as you can to help these poor people. God Bless you. I know it cannot be easy, and it can be very discouraging. I see the results of Bi Polar disorder far more than I would like. Either the suicides, or the mania that goes beyond limits. Sad either way, but it encourages me to know that there are people out there like you who are trying to help them.