She can represent him. It is not a conflict of interest.
How much they make is irrelevant in what you pay for child support. It is irrelevant how much money the step-mother makes since she is not her bio. mother. Parents are required in the State of Texas to support their children. Step-parents are not required to support their step-children.
You are required to pay 20% of your net resources in child support if you have one child. If you have more than one child then you would pay less. You can look on the Texas Attorney General website and calculate what you should be paying. You can "google" Texas child support and there are several websites that "calculate" what you should be paying in child support.
You should also be paying part or all of her health insurance. Health insurance is considered to be an additional form of child support in the State of Texas. Texas now "caps" what you pay in health insurance based on how much money you make. So if you make $20,000 there is a limit to how much you pay in health insurance if the child's health insurance premiums are $500/month.
Lastly, the judge does not care if you "cannot afford to live or not". Child support is your first consideration before you pay rent, food, or shelter in the State of Texas.