In this case you are your brother's keeper, unless you find him appropriate placement. Basically in California, the lease/rental agreements pass from owner to owner until another agreement superceeds it after it expires, there are some statutary expectations. If your mother made an agreement and/or lead your brother to believe that he could live there as long as he wanted and was competent to do so, then that is the agreement, while not a typical rental agreement but does cover her obligation to provide for his care and otherwise he could have contested the will giving you the house. You were given the property with the expectation that you would take care of your brother, this also protects his disability income and the house from SSI obligations. Your, your mother's and brother's conduct demonstrate such an agreement. Your brother is not doing anything to harass you, hurt you, he is complient with therapy, keeps to himself, and you wnat to kick him to the curb?
CALIFORNIA CODES
EVIDENCE CODE
SECTION 620-624
620. The presumptions established by this article, and all other
presumptions declared by law to be conclusive, are conclusive
presumptions.
622. The facts recited in a written instrument are conclusively
presumed to be true as between the parties thereto, or their
successors in interest; but this rule does not apply to the recital
of a consideration.
623. Whenever a party has, by his own statement or conduct,
intentionally and deliberately led another to believe a particular
thing true and to act upon such belief, he is not, in any litigation
arising out of such statement or conduct, permitted to contradict it.
624. A tenant is not permitted to deny the title of his landlord at
the time of the commencement of the relation.
If and or when he is not able to care for himself and requires assisted living contact APS or NAMI for assistance, that is after reasonable measures to assure personal care and hygine, in other words, it isn't illegal to smell or not be as clean as you might like. Help him, find a day program available through mental health department. Please be advised, you could be charged with abuse if you evict him in violation of his rights.