united-states
A person related to you by blood or marriage?
<p>A medical power of attorney form specifies that witnesses to the execution of the power of attorney may not be related by blood or marriage.</p>

<p>Are sons in law and daughters in law included in that restriction?</p>

<p>The point of the restriction is to ensure that witnesses don't have an interest in the issues for which the power of attorney is being granted.  A son in law or daughter in law probably has as much interest or nearly as much interest as his or her spouse, so I would think that those relationships <em>are</em> included in the restriction.</p>

<p>It depends on the state. In Alaska, <a href="http://codes.findlaw.com/ak/title-13-decedents-estates-guardianships-transfers-trusts-and-health-care-decisions/ak-st-sect-13-52-010.html" rel="nofollow noreferrer">AS 13.52.010(e)</a> says</p>

<blockquote>
  <p>At least one of the individuals used as a witness for a durable power
  of attorney for health care shall be someone who is not</p>
  
  <p>(1) related to the principal by blood, marriage, or adoption;  or</p>
  
  <p>(2) entitled to a portion of the estate of the principal upon the
  principal's death under a will or codicil of the principal existing at
  the time of execution of the durable power of attorney for health care
  or by operation of law then existing.</p>
</blockquote>

<p>Washington state only requires the signature of the principle. <a href="http://www.learnprobate.com/wp-content/xlq/displaycodef5b0.html" rel="nofollow noreferrer">California</a> only requires that the witness not be the attorney-in-fact. <a href="http://codes.ohio.gov/orc/1337.12v1" rel="nofollow noreferrer">Ohio</a> requires</p>

<blockquote>
  <p>at least two individuals who are adults and who are not ineligible to
  be witnesses under this division. Any person who is related to the
  principal by blood, marriage, or adoption, any person who is
  designated as the attorney in fact or alternate attorney in fact in
  the instrument, the attending physician of the principal, and the
  administrator of any nursing home in which the principal is receiving
  care are ineligible to be witnesses.</p>
</blockquote>

<p>Notice that "blood, marriage, or adoption" is used in the two states that have relatedness restrictions, and in-laws are related by marriage. </p>
