Legal Question in Real Estate Law in California

Unfortunately ultimately you can't. When multiple people own a property it is as if they each individually own the whole thing by themselves. So she can legally let him do whatever she wants. The only option if the three of you cannot agree on how to deal with him, is to buy her out or sell the house. If the three of you can't agree on that either, the only option is to petition the courts for a "partition." In the old days they literally partitioned property by dividing it up, but with modern zoning and subdivision laws they can't do that. So the court would order the property sold and the proceeds distributed. Usually just filing such an action results in the reluctant party seeing that they are better off just being bought out or agreeing to the sale, rather than spending money in court and having the property sold at a sheriff's sale, which always goes for a lower than market price.