Texas Estates Code - EST § 1104.202. Designation of Guardian for Declarant

(a) A person other than an incapacitated person may designate by declaration a person to serve as guardian of the person or estate of the declarant if the declarant becomes incapacitated.  The court shall appoint the person designated in the declaration to serve as guardian in preference to any other person otherwise entitled to serve as guardian under this title, unless the court finds that the person designated to serve as guardian:

(b) A declarant may, in the declaration, disqualify a named person from serving as guardian of the declarant's person or estate.  The court may not under any circumstances appoint as guardian a person named under this subsection.