Chapter 153 of the Texas Family Code covers Conservatorship, Possession and Access to minor children in the State of Texas.
A minor child is under the age of 18. The moment they turn 18, generally, they are no longer covered under the TX Family Code.
Subchapter H of Chapter 153 addresses the Rights of Grandparent, Aunt or Uncle.
Texas maintains that parents have a fundamental constitutional rights to have and raise children without interference from the state except in extraordinary circumstances (mandatory schooling, mandatory immunization, and abuse or neglect of a child.)
The U. S. Supreme Court case that is often mentioned is Troxel v. Granville, 530 U.S. 57, 120 S.Ct. 2054 decided in 2002.
The Texas legislature in 2009 requires that a grandparent must file an affidavit alleging, along with supporting facts, that denial of the grandparental visitation would significantly impair the child's physical health or emotional well-being. This is a large hurdle to over-come. Merely stating that you do not get along with one of the parent's is not enough. Also, if the grandchild does not "know" the grandparent then it is almost impossible to show that the child would be hurt by having a relationship.
You need an experienced family law attorney if you want to pursue a grandparent, aunt or uncle lawsuit.
Look on www.avvo.com for an attorney or on the State Bar of Texas website - both are free.