Sister Moved Into Elderly Mom’s Rental Without Permission; Now Has A Temporary Restraining Order Against Longtime Tenant

Location: Los Angeles, CA. My mom is 78, retired, and owns a rental property in LA county. Her longtime tenant is my former brother-in-law. He has rented from her for many years without problems, and she depends on his rent to pay her bills. He occupies a separate unit from my mom.
About two years ago, according to what we understand, my sister—his ex-wife—had their daughter copy his house key. While he was at work, she moved into his unit with her younger daughter from another relationship. My understanding is that he did not agree to her moving in. She has remained there since (she’s not on the lease, pay bills/rent).
Since then, he has received repeated verbal abuse and has been picking up extra shifts to avoid being home. He previously requested a restraining order against her, but it was denied for insufficient evidence.
We recently learned that my sister obtained a temporary restraining order against him, alleging that he physically abused their adult daughter, who has autism, and her younger daughter and other lies like him hitting her. There is an upcoming hearing, and I have not seen the full order myself.
My sister has also repeatedly argued with my mom. I’m my mom’s only caregiver, and my sister previously reported me for elder abuse, an allegation I dispute. I’m including that because there is significant family conflict involved and everyone avoids her like a plague cause how problematic she is. My mom wants my sister to leave the rental. The tenant is considering leaving himself because of the situation, which could leave my mom without the rental income she relies on while my sister remains. We all believe she has some sort of mental problems but she declines help. What should we do in this situation.