What can I do for a woman (88) who is angry, lacks trust, forgetful and extremely paranoid?

My mother in law has a very difficult mother. She is a 88 year old women who gets worked up about everything. She does not trust anyone not even her own daughter anymore. She thinks that a group of women that visited her late husband comes in her house and steals her things. She looses those things and blame them when it clearly is she can't remember where she put it or has way too much of it. Clothes for example. She even thinks they could have killed her husband. She is very demanding and ungrateful. I know that a lot of it has to do with aging but where do you draw the line? My mother in law is absolutely clueless on what to do but continues run ragged for her mom. I definitely admire her for that but feels like something needs to be done. My mother in law's mom don't think anything is wrong so meds are not an option. Plus she probably wouldn't trust anyone with something like that. My mother in law asked if I could go once week so she could get a break to help. I am more than willing. I just don't know completely how to help.