Legal Question in Real Estate Law in California

Not much unless you want to force a sale of the property to a third party, or force a buy-out. They were within their rights as joint tenants.

Joint tenants can lease to others, but no more than they have. Co-owners of real property are, in a sense, roommates. Each has the right to shared possession of every square inch of the entire property at every minute. However, this right carries a limitation, because if one co-owner has come into full possession somehow, and doesn't want to let the other(s) back in when they demand restoration of shared possession, we have what is called an "ouster," and although the ouster is unlawful, so is trying to regain possession by self-help, because that usually leads to a breach of the peace. So, the law has a procedure for curing an ouster and getting the other co-owners back in peaceable possession, perhaps with the sheriff's help. See Civil Code section 843.

Co-owners may also alter their rights to concurrent possession by contract between themselves.

So, if A, B and C are the family members owning the property in joint tenancy, and C rents to X, a stranger, A and/or B may sue C and X for shared possession. X will then have A and B as roommates, so to speak.

Mr. McCormick is more or less correct to say C was "within his rights" as the co-owner in possession of the property to rent it to X. However, he has no right to compromise or deny A and B their rights, and A and/or B can make life difficult for C and X if they choose to pursue remedies for ouster.

C must also share any profit he makes from renting to X with his co-owners, A and B.

The exit strategy for unhappy co-owners is a special kind of lawsuit called "partition," which asks a court to divide the property, either by actual subdivision or more often in modern times by sale and equitable allocation of the net proceeds of sale among the erstwhile co-owners.