Cal. PROB § 8923

The court may not designate as probate referee any of the following persons: (a) The court clerk. (b) A partner or employee of the judge or commissioner who orders the designation. (c) The spouse of the judge or commissioner who orders the designation. (d) A person, or the spouse of a person, who is related within the third degree either (1) to the judge or commissioner who orders the designation or (2) to the spouse of the judge or commissioner who orders the designation.