Texas Estates Code - EST § 1023.001. Venue for Appointment of Guardian

(1) in the county in which both the minor's parents reside;

(2) if the parents do not reside in the same county, in the county in which the parent who is the sole managing conservator of the minor resides, or in the county in which the parent who is the joint managing conservator with the greater period of physical possession of and access to the minor resides;

(3) if only one parent is living and the parent has custody of the minor, in the county in which that parent resides;

(4) if both parents are dead but the minor was in the custody of a deceased parent, in the county in which the last surviving parent having custody resided;  or

(5) if both parents of a minor child have died in a common disaster and there is no evidence that the parents died other than simultaneously, in the county in which both deceased parents resided at the time of their simultaneous deaths if they resided in the same county.