I am in PA. My sister had a very brief affair that resulted in a pregnancy and her claiming R was the father of her son. R claimed doubts about paternity and did not want anything to do with my sister or the child (if it was his or not). My sister, for whatever reasons, put unknown father on the child's BC (child was born in Florida). The supposed father, R, has not attempted to get a paternity test or see the child, and has no contact with my sister or my nephew. My sister has since moved back to PA. I'm wondering if my sister or her son can sue for back child support at any time even if she put unknown father on the BC and no paternity test has been done?