Washington Revised Code Title 11. Probate and Trust Law § 11.20.040. Proof where one or more witnesses are unable or incompetent to testify, or absent from state

The subsequent incompetency from whatever cause of one or more of the subscribing witnesses, or their inability to testify in open court or pursuant to commission, or their absence from the state, shall not prevent the probate of the will. In such cases the court shall admit the will to probate upon satisfactory testimony that the handwriting of the testator and of an incompetent or absent subscribing witness is genuine or the court may consider such other facts and circumstances, if any, as would tend to prove such will.