Grandparents do have rights in Texas, under certain fact intensive situations. First, the question is whether you are seeking access of your son-in-law or to be named his managing conservator. Traditionally, Texas grandparents may file an original suit only if seeking managing conservatorship; whereas, if seeking possession or access to your grandchild, you may only intervene in a pending suit. In either situation, a grandparent's standing is a difficult issue to overcome for many reasons not addressed herein.
If you are seeking possession or access, you may only intervene in an original suit or a suit for modifying the parent-child relationship. A grandparent in general may file suit affecting the parent-child relationship, if among other things, has had actual control, care, and possession of the child for at least 6 months (which is not the case here, but is given for an example only). A grandparent may also file suit for managing conservatorship if the court finds the present circumstances are harmful to the child. In either situation, it seems that Texas Courts have heavily put the burden on grandparents to prove their position.
At the end of the day, however, Texas Courts also look to the best interest of the child. This is the primary consideration whether a grandparent is seeking possession and/or managing conservator.
This is just an opinion, and is no way to take the place of an attorney that knows all the facts of the case. Therefore, I would seek such advice as soon as possible.
Sign up to receive a 3-part series of useful information and advice about child custody law.