I'll see what I can provide for you but it seems like you need the intervention of the court in order to establish custody. With regard to which court, here is the relevant statute:
Texas Family Code
§ 103.001. VENUE FOR ORIGINAL SUIT.
(a) Except as otherwise provided by this title, an original suit shall be filed in the county where the child resides, unless:
(1) another court has continuing exclusive
jurisdiction under Chapter 155; or
(2) venue is fixed in a suit for dissolution of a
marriage under Subchapter D, Chapter 6.
(b) A suit in which adoption is requested may be filed in the
county where the child resides or in the county where the
petitioners reside.
(c) A child resides in the county where the child's parents
reside or the child's parent resides, if only one parent is living,
except that:
(1) if a guardian of the person has been appointed by
order of a county or probate court and a managing conservator has
not been appointed, the child resides in the county where the
guardian of the person resides;
(2) if the parents of the child do not reside in the
same county and if a managing conservator, custodian, or guardian
of the person has not been appointed, the child resides in the
county where the parent having actual care, control, and possession
of the child resides;
(3) if the child is in the care and control of an adult
other than a parent and a managing conservator, custodian, or
guardian of the person has not been appointed, the child resides
where the adult having actual care, control, and possession of the
child resides;
(4) if the child is in the actual care, control, and
possession of an adult other than a parent and the whereabouts of
the parent and the guardian of the person is unknown, the child
resides where the adult having actual possession, care, and control
of the child resides;
(5) if the person whose residence would otherwise
determine venue has left the child in the care and control of the
adult, the child resides where that adult resides;
(6) if a guardian or custodian of the child has been
appointed by order of a court of another state or country, the child
resides in the county where the guardian or custodian resides if
that person resides in this state; or
(7) if it appears that the child is not under the
actual care, control, and possession of an adult, the child resides
where the child is found.