Caring for two mothers, one my mother-in-law and the other my mom. Any advice?

I haven't been on this site in many months. My mother in law lives with my wife and I and is in late stage dementia. Some days we get through without a lot of issues and others when she is sundowning can be trying. We have help for part of each day, but the rest falls on my wife for the most part and myself. The other half of the equation is my 101 year old mother. My dad passed away about a year ago. My mother can be very manipulative. She refused to move in with us and will not go to assisted living. I have a girl who is with her four mornings a week and I am with her seven days a week from lunch through dinner. My parents were married 72 years. My mother can't or won't let go of the fact that my dad is gone. She cries most days, has some dementia , and is just a handful. Sure there are some good days, but the bad ones seem to outweigh the good days. A typical day finds her crying, then humming a song she says my dad loved. She doesn't know what day, month, year it is nor does she care. She can be delusional at times. She uses a walker but at times will just say I am not using it. One of the things that drive me crazy is when she will accuse me or the young lady helping of stealing from her. A month ago she said we stole her bracelet. I said to her your bracelet is where you left it on your bureau. Today she said I took her adding machine. I was totally baffled and said what adding machine. She said the one your father used when in business. I said mom, that was fifty years ago. At any rate she was fixated on the adding machine. I attempt to compartmentalize whatever I am dealing with when with her and then whatever is going on at home with my mother in law. My wife and I have been dealing with her mother for many years and my mom since my dad died without any break. We are both walking on eggshells.