Tex. Health and Safety Code § 166.039

(a) If an adult qualified patient has not executed or issued a directive and is incompetent or otherwise mentally or physically incapable of communication, the attending physician and the patient's legal guardian or an agent under a medical power of attorney may make a treatment decision that may include a decision to withhold or withdraw life-sustaining treatment from the patient.

(b) If the patient does not have a legal guardian or an agent under a medical power of attorney, the attending physician and one person, if available, from one of the following categories, in the following priority, may make a treatment decision that may include a decision to withhold or withdraw life-sustaining treatment:

(1) the patient's spouse;

(2) the patient's reasonably available adult children;

(3) the patient's parents; or

(4) the patient's nearest living relative.

(c) A treatment decision made under Subsection (a) or (b) must be based on knowledge of what the patient would desire, if known.

(d) A treatment decision made under Subsection (b) must be documented in the patient's medical record and signed by the attending physician.

(e) If the patient does not have a legal guardian and a person listed in Subsection (b) is not available, a treatment decision made under Subsection (b) must be concurred in by another physician who is not involved in the treatment of the patient or who is a representative of an ethics or medical committee of the health care facility in which the person is a patient.

(f) The fact that an adult qualified patient has not executed or issued a directive does not create a presumption that the patient does not want a treatment decision to be made to withhold or withdraw life-sustaining treatment.

(g) A person listed in Subsection (b) who wishes to challenge a treatment decision made under this section must apply for temporary guardianship under Chapter 1251, Estates Code. The court may waive applicable fees in that proceeding.