Mother in-law problems

My mother-in-law has a house that she inherited and she rents it to my husband and I . We've been living here for 4 years and its becoming annoying. No she doesn't live with us but she wants to lol Her and her husband often get in arguments and every year she threatens to leave him and move in with us. Because she technically owns the house she can do what she wants. But there's a way to do things. The agreement was that we pay her full rent but if we come up short she would understand and give us time to pay it. We've been short lately so shes all of the sudden fighting with her husband and says that she moving in and theres nothin we can do about it. My husband is so fed up but we can't afford to move rite now. They fuss all the time and I hate to see him and his mom fuss but shes crazy . How can I keep my sanity in this situation.

Ps. She also feels that because its her house she can pop up unannounced and expect for us to run errans for her.