I don't think you are referring to the document your sister-in-law received by the correct name. If she received a general denial, that would mean she (your sister-in-law) has filed a petition and her son's father was named as the respondent (ie - the other party in the lawsuit). A general denial is just the name given to the response that he is required to file with the court. You sister-in-law probably filed a motion to enforce child support because her son's father is delinquent on child support payments.
However, without knowing all the details, it's hard to tell you exactly what to expect next. Definitely contact an attorney about this. Best of luck.
This answer is intended for informational and educational purposes only and should not be considered legal advice nor constituting an attorney-client relationship. This attorney is licensed in Texas.
Also, if the child support office is pursuing the claim and he files a general answer, your sister-in-law is entitled notice of the answer (and hence a copy) as a party to the lawsuit. He is simply denying everything so that he cannot be defaulted. That means there is going to need to be a hearing unless he will agree to an order.
This answer is intended for informational and educational purposes only and should not be considered legal advice nor forming the attorney client relationship. This attorney is licensed in Texas.