Legal Question in Family Law in Texas

Neither a parent, stepparent, or grandparent would have custody of a 20 year old. A 20 year old is an adult, and not subject to being in anyone's custody. The only possible exception to that would be if there were some sort of severe physical or mental disability that required establishment of a guardianship, but you don't refer to that as "custody", and it works very differently. If that's what you mean, yes, there's a way to request transfer of an existing guardianship to a more appropriate person if there's a problem with the original guardian.