Texas Estates Code - EST § 1104.051. Guardian of Minor Children

(a) If the parents live together, both parents are the natural guardians of the person of the minor children by the marriage, and one of the parents is entitled to be appointed guardian of the children's estates.  If the parents disagree as to which parent should be appointed, the court shall make the appointment on the basis of which parent is better qualified to serve in that capacity.

(b) The rights of parents who do not live together are equal.  The court shall assign the guardianship of their minor children to one parent considering only the best interests of the children.

(c) If one parent is deceased, the surviving parent is the natural guardian of the person of the minor children and is entitled to be appointed guardian of the minor children's estates.