Asalamualikum, I was taught in many fiqh classes that when someone marries a girl or a boy the father in law and mother in law become Mahram after marriage consummation. If someone marries a woman or a man their daughters and sons from previous marriage become Mahram right away. Now when I read in many fatawas, they all become Mahram before marriage consummation. Would you pleas tell me which one is correct or there is difference of opinion among scholars. Jazakum Allah Khairan

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The parents and grandparents of the husband and his children and grandchildren become forbidden for his wife as soon as the marriage contract is conducted, and the mother and grandmothers of the wife also become forbidden for her husband as soon as the marriage contract is conducted. As regards the daughters (and granddaughters) of the wife, then they do not become forbidden for her husband until the marriage is consummated.

Therefore, if a man marries a woman, then her mother becomes forbidden for him as soon as the marriage contract is conducted, according to the view of the majority of the scholars among whom are the Four Schools of jurisprudence. This is also the preponderant opinion as Allaah Says when mentioning the women whom we are forbidden to marry (what means): {…and your wives’ mothers…}[Quran 4:23] Allaah did not condition the consummation of the marriage for the prohibition of the marriage of the mother of the wife to the wife’s husband. Contrarily to the opinion of the majority of the scholars Ibn Qudaamah said: “It was reported from ‘Ali (Ibn Abi Taalib) that she [the mother of the wife] does not become forbidden unless one consummates the marriage with her daughter, in the same manner her daughter does not become forbidden to the husband unless the marriage [with the wife] is consummated.” For more benefit, please refer to Fatwa 85868.

The wife also becomes forbidden to her husband’s father as soon as the marriage contract takes place; Allaah Says (what means): {And [also prohibited are] the wives of your sons who are from your [own] loins.}[Quran 4:23] 

Ibn Qudaamah also said: “It is forbidden for a husband to marry the wives of his sons as well as the wives of the sons of his granddaughters, either from family lineage or from suckling relationship, regardless whether the relation is close or remote, as soon as the marriage contract is conducted, as Allaah Says (what means): {And [also prohibited are] the wives of your sons who are from your [own] loins.}[Quran 4:23], and we do not know any difference of opinion about this.”

Allaah Knows best.