“I am married and have two children with my husband. We are now separated because of an affair my husband had with a girl that is now saying he is the father of her son. What can I do ask his wife to demand a DNA test? Do i have that right?”

Not directly. You personally are not implicated in his paternity action and cannot demand a DNA test for the purpose of establishing he is the father of that child. In a dissolution of marriage case in a fault-divorce jurisdiction, it may be relevant to establishing adultery if he does not admit it. Under that circumstance, after discovery where you ask him to admit or deny extra-marital affairs, you might then seek a subpoena to draw blood from him and the child to prove his parentage. This may be too invasive of the child’s rights, and would certainly take longer than the 6 months waiting period to get a divorce n most states. Since you already apparently have proof he committed adultery, you may likely never be able to get that other child tested.