What are the legal consequences when a man's son marries a woman whose daughter the man also married, resulting in each woman being given to the other man, and consummation occurs?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
If the first man (the father-in-law) consummates the marriage with the woman married to his son, this act necessitates for him the dower (mahr) equivalent to her customary dower due to the act being one of dubious legal basis (shubha). This consummation invalidates her marriage to her husband (the son) because she has become related through marriage (halila) to the son or father. The son's right to the dowry is nullified because the invalidation originated from the wife's action through permitting and cooperating in the consummation. The man who performed the act owes nothing to the husband because no liability exists that allows for recourse. Furthermore, the wife shares in causing the invalidation of her marriage through her compliance, so the husband owes nothing, similar to a case where she alone causes it. If the first act is ambiguous or unclear, both marriages are voided, and each man owes the customary dower to the woman he defiled. Recourse between the two defilers is not established, and each man owes the other's wife half of the stipulated dower, which is not waived by doubt.
Supporting text
A possibility exists that the defiler owes the husband half of her customary dower because he corrupted the marriage before consummation, resembling a case where a wife corrupts a marriage through fostering (ridaa). Additionally, the defiler's own marriage is also dissolved because his wife has become either a mother-in-law or daughter-in-law to the woman he defiled, and he owes her half of the stipulated dower.