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Really the only one who knows the answer to that is the sister-in-law. No one knows what promises she may have made to her sister before she died. One tries to honor such promises even when it is unpleasant to do so.
I know in my own family, I promised my sister on her death bed that I would do whatever I could to look after her husband, and I've tried to do that. There wasn't any question of his mistreating her as it sounds like might have happened in your family, but it is a promise I made, even when it isn't really convenient to keep. Thank goodness my husband understands the commitment and supports me.
I would suggest you discuss this with the lady in question, since she is the only one who has the answer.
I'm sorry you think I'm biased. I'm not. I was just giving an example of the kind of reason the sister-in-law might have. The only one who knows why she takes on the responsibility for helping her brother-in-law is her.
Have you asked her why she does it?
That makes sense. If she promised her sister to care for the sister's husband, then she is just fulfilling her promise as honorably as she can. That can be inconvenient, and difficult, but it is a promise. Hopefully, it comforts you to know she is a person of her word. Perhaps she can cut down on the number of phone calls he makes, but he is probably mourning as well. Even when people mistreat their mates, they may still love them on some level, and grieve over their deaths.