Your mother-in-law is lonely. I presume from your story that she has no husband, as you don't mention one. Maybe if she had some company her own age, or a boyfriend, she might stay home to visit with them. Does she even still have her own home? It may be an economic issue, and she's too embarrassed to let you know she has no where else to go. Visiting her daughter and taking care of you could be her excuse to stay there without feeling like a mooch.
Having a frank discussion with her shouldn't hurt her feelings. I'm sure she had her own difficulties with her mother-in-law and can see the position she's put you in. Maybe if she has a problem and you can solve it, you'll have peace.