Tex. Property Code § 114.032

(a) A written agreement between a trustee and a beneficiary, including a release, consent, or other agreement relating to a trustee's duty, power, responsibility, restriction, or liability, is final and binding on the beneficiary and any person represented by a beneficiary as provided by this section if:

(1) the instrument is signed by the beneficiary;

(2) the beneficiary has legal capacity to sign the instrument; and

(3) the beneficiary has full knowledge of the circumstances surrounding the agreement.

(b) A written agreement signed by a beneficiary who has the power to revoke the trust or the power to appoint, including the power to appoint through a power of amendment, the income or principal of the trust to or for the benefit of the beneficiary, the beneficiary's creditors, the beneficiary's estate, or the creditors of the beneficiary's estate is final and binding on any person who takes under the power of appointment or who takes in default if the power of appointment is not executed.

(c) A written instrument is final and binding on a beneficiary who is a minor if:

(1) the minor's parent, including a parent who is also a trust beneficiary, signs the instrument on behalf of the minor;

(2) no conflict of interest exists; and

(3) no guardian, including a guardian ad litem, has been appointed to act on behalf of the minor.

(d) A written instrument is final and binding on an unborn or unascertained beneficiary if a beneficiary who has an interest substantially identical to the interest of the unborn or unascertained beneficiary signs the instrument. For purposes of this subsection, an unborn or unascertained beneficiary has a substantially identical interest only with a trust beneficiary from whom the unborn or unascertained beneficiary descends.

(e) This section does not apply to a written instrument that modifies or terminates a trust in whole or in part unless the instrument is otherwise permitted by law.