Tex. Health and Safety Code § 166.003

In any circumstance in which this chapter requires the execution of an advance directive or the issuance of a nonwritten advance directive to be witnessed:

(1) each witness must be a competent adult; and

(2) at least one of the witnesses must be a person who is not:

(A) a person designated by the declarant to make a health care or treatment decision;

(B) a person related to the declarant by blood or marriage;

(C) a person entitled to any part of the declarant's estate after the declarant's death under a will or codicil executed by the declarant or by operation of law;

(D) the attending physician;

(E) an employee of the attending physician;

(F) an employee of a health care facility in which the declarant is a patient if the employee is providing direct patient care to the declarant or is an officer, director, partner, or business office employee of the health care facility or of any parent organization of the health care facility; or

(G) a person who, at the time the written advance directive is executed or, if the directive is a nonwritten directive issued under this chapter, at the time the nonwritten directive is issued, has a claim against any part of the declarant's estate after the declarant's death.