Washington Revised Code Title 11. Probate and Trust Law § 11.12.160. Interested witness--Effect on will

(1) An interested witness to a will is one who would receive a gift under the will.

(2) A will or any of its provisions is not invalid because it is signed by an interested witness. Unless there are at least two other subscribing witnesses to the will who are not interested witnesses, the fact that the will makes a gift to a subscribing witness creates a rebuttable presumption that the witness procured the gift by duress, menace, fraud, or undue influence.

(3) If the presumption established under subsection (2) of this section applies and the interested witness fails to rebut it, the interested witness shall take so much of the gift as does not exceed the share of the estate that would be distributed to the witness if the will were not established.

(4) The presumption established under subsection (2) of this section has no effect other than that stated in subsection (3) of this section.