What is the ruling on a man marrying a woman who is already married to another man, knowing this, and does the state of mahramiyyah become established between his children from her and his children from another wife, and are his grandchildren from the two women considered paternal and maternal uncles and aunts?

Among the conditions for the validity of a marriage is that the wife be free from legal impediments. Therefore, it is not permissible to marry a woman who is already married, based on the Almighty's saying: "And all married women." The marriage in this case is null and void and must be annulled. If it was conducted with both parties aware of the prohibition, they are liable to the prescribed punishment (hadd). If a child is born from this void marriage, he is not attributed to the adulterer, and the rulings of filiation do not apply to him. Rather, he is attributed to the woman's legal husband, based on the Prophet's (peace be upon him) saying: "The child is for the bed (legal marriage), and for the adulterer is the stone (punishment/deprivation)." His lineage is not denied from the husband except through mutual imprecation (li'an).