It depends, where you are from, it also depends the time period. I would try to find the Marriage record, on Some Marriage records they requird the names of the Parents, I also would look for the Divorce Record, in some place the did the same, Farther more you could also find information on the other child and the birth and marriage records, because it would have names on them also. Death Records also would have these things too. Those are the Common Records for stuff, if you don't find it there than you will have to look even harder, but I would start their. Good luck I hope the best.