Is the marriage of a daughter to her cousin valid if it is later discovered that the girl's father committed incest with the husband's mother? And what are the implications of that for the couple and the child?

Scholars have differed on whether adultery spreads حرمة (prohibition of marriage) or not. The relied-upon opinion is that it does not spread it; rather, its fluid (semen) is considered waste and is not given اعتبار (legal consideration). Based on this, the fact that the girl's father committed adultery with her husband's mother does not affect the validity of the children's marriage. They should seek forgiveness for their parents and conceal their secret.