My x wife has requested through the ag's office in Texas for me to relinquish rights to the 2 girls, but I know that I am not the bio father what do I do next and am I still held accountable for the child support.

Take a paternity test and with her trying to relinquish rights, if the tests prove you are not the father it should work out in your favor hopefully.

Were you married when she had the children ? If so you can be considered ( in the eyes of the law ) as the legal father.