Witnesses, Generally
Pennsylvania requires two subscribing witnesses to every living will.  If the principal personally signs the document, a witness may be any competent person who is at least eighteen years old.

If the principal must ask another person to sign on his or her behalf any competent adult person is permitted to sign as a witness, except for the person who writes the principal's signature to the document.

In either case, the witnesses must be present and observe the principal apply his or her signature to the document or observe the signature being made on the principal's behalf.

A Pennsylvania living will that has not been properly witnessed is invalid and will not be followed.

See: Signing By Another Person