Texas law makes it:

A first-degree felony for marrying a child younger than 16 and a second-degree felony for marrying a child 16 and 17.

A class A misdemeanor for a person to give consent for an underage marriage if they are not the parent, managing conservator or guardian of the applicant

A third-degree felony for a person to who knowingly conducts a marriage ceremony of a minor whose marriage is prohibited by law

A class A misdemeanor for knowingly providing false, fraudulent or otherwise inaccurate information regarding identity or age under this section

Illegal to marry a former or current stepchild, stepparent or cousin by blood or by adoption

So a person cannot be a party to an informal marriage or execute a declaration of an informal marriage if the person is presently married to a person who is not the other party to the informal marriage

Source: Texas Legislature analysis