Assalamualaikum ustaz. Can my father marry my mother-in-law? Hope for an answer.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Mahram is an individual who is prohibited to be married whether it is due to a womb or familial relationship. It can also be defined as something that is prohibited by Allah SWT.
Mahram according to terminology explained by jurists means: “A person who one is prohibited to marry and its prohibition may be in the form of al-ta’bid (forever) with mubah reasons for the person’s honour and not because there is a barrier that may not exist any longer.” 
The explanation regarding the group of people that is considered as a person’s mahram is described in the statement of Allah SWT:
حُرِّمَتْ عَلَيْكُمْ أُمَّهَاتُكُمْ وَبَنَاتُكُمْ وَأَخَوَاتُكُمْ وَعَمَّاتُكُمْ وَخَالَاتُكُمْ وَبَنَاتُ الْأَخِ وَبَنَاتُ الْأُخْتِ وَأُمَّهَاتُكُمُ اللَّاتِي أَرْضَعْنَكُمْ وَأَخَوَاتُكُم مِّنَ الرَّضَاعَةِ وَأُمَّهَاتُ نِسَائِكُمْ وَرَبَائِبُكُمُ اللَّاتِي فِي حُجُورِكُم مِّن نِّسَائِكُمُ اللَّاتِي دَخَلْتُم بِهِنَّ فَإِن لَّمْ تَكُونُوا دَخَلْتُم بِهِنَّ فَلَا جُنَاحَ عَلَيْكُمْ وَحَلَائِلُ أَبْنَائِكُمُ الَّذِينَ مِنْ أَصْلَابِكُمْ وَأَن تَجْمَعُوا بَيْنَ الْأُخْتَيْنِ إِلَّا مَا قَدْ سَلَفَ ۗ إِنَّ اللَّهَ كَانَ غَفُورًا رَّحِيمًا
“Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives’ mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.” 
Imam al-Nawawi when explaining who is meant by the phrase “your sisters” in the above verse said: “Your sisters (of whom you’re prohibited from marrying) are those who are the daughters of your parents or one of them.” 
Imam al-Mawardi cited the words of Imam al-Syafie saying: “The origin that makes a woman a mahram are two reasons. One is the ansab (lineage or family ties) and the next is asbab (reasons) that may be a result of marriage or nursing (suckling).”
Imam al-Mawardi then further explained this saying: “A mahram woman are categorized into two. First: The first part where she is prohibited forever. While the other part is that she is prohibited if gathered (at the same time). And the part of which a woman is a mahram forever is further divided into two which are nasab and asbab (due to marriage and suckling).” 
According to the above question, the marriage between the in-laws is valid and permissible regardless of whether their children is still married or have divorced. Likewise, this marriage is valid and permissible whether your father is still married or has divorced.
We give the situational example here:
- Situation 1: Ahmad and Zainab got married. Then Ahmad’s father, Marwan (the wife’s father-in-law) marries Zainab’s mother Fatimah (the husband’s mother-in-law).
- Situation 2: Marwan and Fatimah (Ahmad’s Father and Aminah’s mother) got married. After that, Ahmad and Aminah (their stepdaughter).
This issue is the same with step-siblings of different mothers and fathers, they are not mahram and can be married.
While the daughter of the stepmother, Imam Nawawi continues: “It is not mahram for a daughter of a step-father, mother to a step-father, step-child of a daughter, mother to a son-in-law, mother to mother-in-law, and it is also not mahram for a daughter of step-mother (who isn’t born as a result of the marriage with his father).” 
Syeikh al-Bujairimi said: “Indeed, it is not prohibited to marry the daughter of his mother’s husband (stepsisters), it is also not prohibited with the mother of his mother’s husband (step-grandmother), it is also not prohibited with the daughter of the husband’s daughter (step-daughter to the child), it is also not with the mother of the child’s husband (the child’s in-law), not prohibited for the mother of the wife’s father (step-grandmother), also not prohibited daughter of father’s wife (step-sisters), not prohibited, mother of wife’s child (child’s in-law), also not prohibited with the child of the wife’s child (daughter of step-child) and also not prohibited with the wife of a stepchild for they aren’t included as those stated as prohibited to be married.” 
While for their status in terms of family bonds, they are subject to the rulings set by syara’ such as in matters related to mahram, inheritance, wali and others. Lastly, may Allah SWT grant us all the correct understanding in the matters of religion. Amin.