You really need to know for sure what the law dictates in Texas. I do know that many states do not have clear laws. If I am remembering correctly, one poster mentioned that Florida is one state for sure where they do not recognise 'grandparents rights'.
I would ask your lawyer to give you the actual reference to Texas legal code where grandparents rights, or lack thereof, are dictated.
I am thinking that you should be documenting everything that has happened with your inlaws. Some of the things that they have done are illegal, and would constitute child endangerment or neglect. If these things are well documented, then you would have just cause to request that the inlaws never be granted unsupervised visits with the children. Don't many child custody/visitation agreements have stipulations like this? (No unauthorized unknown adults residing in the home, etc.. etc...)
Man, you don't even want to get me started on grandparents rights!!! Once a person has turned 18 years of age and left home, their rights are OVER!!!" Meddling inlaws and parents are vile enough without giving them any legal encouragment!!!