Cal. Probate Code § 6112

(a) Any person generally competent to be a witness may act as a witness to a will. (b) A will or any provision thereof is not invalid because the will is signed by an interested witness. (c) Unless there are at least two other subscribing witnesses to the will who are disinterested witnesses, the fact that the will makes a devise to a subscribing witness creates a presumption that the witness procured the devise by duress, menace, fraud, or undue influence. This presumption is a presumption affecting the burden of proof. This presumption does not apply where the witness is a person to whom the devise is made solely in a fiduciary capacity. (d) If a devise made by the will to an interested witness fails because the presumption established by subdivision (c) applies to the devise and the witness fails to rebut the presumption, the interested witness shall take such proportion of the devise made to the witness in the will as does not exceed the share of the estate which would be distributed to the witness if the will were not established. Nothing in this subdivision affects the law that applies where it is established that the witness procured a devise by duress, menace, fraud, or undue influence.