Legal Question in Real Estate Law in California

Do you mean if the sick sibling dies?

You are short on information, such as how title is held to the property, and whether the sick sibling has a will or a trust.

As long as the sick sibling is alive, his share goes to him. If he is so sick he needs is affairs managed, he would need to have a conservator appointed to handle his money and related obligations. Once he dies, his estate would become the owner of his share and any proceeds of sale would go according to his will, trust, or intestate succession laws.

Without disputing the correctness of the preceding answers, I feel it should be added that what happens to property ownership when one of two or more owners dies depends upon how the co-owners hold title. If they are tenants in common, the ownership interest of the deceased co-owner will go to his (or her) heirs under his/her will, trust, or according to the laws of intestate succession. If the co-owners hold title as joint tenants, the deceased owner's share "disappears" and the percentage ownership of the remaining owner(s) increase(s) accordingly, and the heirs get nothing.