Wash. Rev. Code § 68.50.101

Autopsy or postmortem may be performed in any case where authorization has been given by a member of one of the following classes of persons in the following order of priority:

(1) The surviving spouse or state registered domestic partner;

(2) Any child of the decedent who is eighteen years of age or older;

(3) One of the parents of the decedent;

(4) Any adult brother or sister of the decedent;

(5) A person who was guardian of the decedent at the time of death;

(6) Any other person or agency authorized or under an obligation to dispose of the remains of the decedent. The chief official of any such agency shall designate one or more persons to execute authorizations pursuant to the provisions of this section.

If the person seeking authority to perform an autopsy or postmortem makes reasonable efforts to locate and secure authorization from a competent person in the first or succeeding class and finds no such person available, authorization may be given by any person in the next class, in the order of descending priority. However, no person under this section shall have the power to authorize an autopsy or postmortem if a person of higher priority under this section has refused such authorization: PROVIDED, That this section shall not affect autopsies performed pursuant to RCW 68.50.010 or 68.50.103.