Assalamualaikum ustaz. Is it permissible for me to marry my wife’s aunt? If it is prohibited is it a permanent or temporary prohibition?
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 according to the jurists means: “A person who is prohibited for us to marry and the prohibition may be in the form of al-ta’bid (permanent/forever) due to mubah reasons and not because of honour nor is it because there is something that prevented us from doing so and could be gone.” 
The explanation regarding people who are considered a person’s mahram is stated 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.” 
The prohibition of marrying certain women is divided into two. The first is Muabbad which is the eternal prohibition from marrying her forever. And the second is tahrim muaqqat which means the prohibition from marrying her for a temporary time due to factors that prevented him. If one still marries the prohibited woman, then it is a false and invalid marriage.
Al-Quran didn’t explain the ruling of marrying the aunt of one’s wife. What is explained in al-Quran is the prohibition of marrying two sisters concurrently. Allah SWT states:
وَأَن تَجْمَعُوا۟ بَيْنَ ٱلْأُخْتَيْنِ إِلَّا مَا قَدْ سَلَفَ ۗ
“[it is prohibited] and that you take [in marriage] two sisters simultaneously,” 
However, this ruling is detailed by the Prophet PBUH in a narration from Jabir:
نَهَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنْ تُنْكَحَ الْمَرْأَةُ عَلَى عَمَّتِهَا أَوْ خَالَتِهَا
“The Messenger of Allah forbade taking a woman as a co-wife to her paternal aunt or her maternal aunt.” 
Ibn Hajar al-Asqalani stated that no scholars disagree with the prohibition of marrying both a woman and her aunt at the same time. 
In another narration from Abu Hurairah, the Prophet PBUH detailed this ruling further. The Prophet PBUH said:
لَا تُنْكَحُ الْمَرْأَةُ عَلَى عَمَّتِهَا وَلَا الْعَمَّةُ عَلَى بِنْتِ أَخِيهَا وَلَا الْمَرْأَةُ عَلَى خَالَتِهَا وَلَا الْخَالَةُ عَلَى بِنْتِ أُخْتِهَا وَلَا تُنْكَحُ الْكُبْرَى عَلَى الصُّغْرَى وَلَا الصُّغْرَى عَلَى الْكُبْرَى
“A woman should not be married to one who had married her paternal aunt or a paternal aunt to one who had married her brother’s daughter or a woman to one who had married her maternal aunt or maternal aunt to one who had married her sister’s daughter. A woman who is elder (in relation) must not be married to one who had married a woman who is younger (in relation) to her nor a woman who is younger (in relation) must be married to one who has married a woman who is elder (in relation) to her.” 
According to both these narrations, it is clear that it is prohibited to marry a wife’s aunt. However, is it a muabbad prohibition which means that it is a permanent prohibition? Scholars agreed that the prohibition isn’t a muabbad prohibition however, it is categorized as muaqqat which is temporary for as long as there are certain factors that prohibited him from doing so. This means the wife’s aunt is prohibited for as long a person’s relationship as husband and wife still exist.
A famous Islamic legal maxim stated by scholars said:
يَحْرُمُ الْجَمْعُ بَيْنَ كُلِّ امْرَأَتَيْنِ بَيْنَهُمَا قَرَابَةٌ وَلَوْ كَانَتْ إِحْدَاهُمَا رَجُلًا لَم يجز لَهُ الزَّوَاج بِالأُخْرَى
“Prohibited to gather (marrying both at the same time) two women who have a familial relationship. If one of them is married, the other is prohibited from marrying.”
However, if the wife has passed away or divorced and completed her iddah, then at this time, it is permissible to marry the ex-wife’s aunt since the mahram muaqqat (temporary mahram) has ended.
The wisdom behind this prohibition is explained by the Prophet PBUH in a ahdith narrated by Ibn Abbas R.Anhuma, he said:
أَنّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّم َنَهَى أَنْ تُزَوَّجَ الْمَرْأَةُ عَلَى الْعَمَّةِ وَعَلَى الْخَالَةِ، وَقَالَ: إِنَّكُمْ إِنْ فَعَلْتُمْ ذَلِكَ قَطَعْتُمْ أَرْحَامَكُمْ
“Indeed, Rasullullah PBUH prohibited a man from marrying a woman together with her paternal or maternal aunt. The Prophet PBUH said: If you did as such, you have severed your bonds (family relations).” 
It means the prohibition if violated will cause chaos and disharmony in terms of familial relationships.
Syeikh al-Maraghi said: “The illah on the prohibition is because it may sever the silaturrahim of which Allah has commanded to be maintained.” 
In truth, the prohibition is to maintain familial relationships. Islam wants to prevent any jealousy, envy, enmity and hatred in families.
To conclude, we state that it is impermissible for a person to marry his wife’s aunt as long as he is still married to his wife. If the wife passed away or divorced and has completed her iddah, then only at this time that the aunt of the ex-wife is permissible for the person. May Allah grant us all the correct understanding of this religion.
 See Kasyifah al-Saja, hlm. 114
 Surah Al-Nisa’: 23
 Narrated by al-Bukhari (5108) and Muslim (3506)
 See Fath al-Bari, 25/290
 Narrated by al-Tirmizi (1045), Abu Dawud (2065) and al-Nasaie (3294)
 Narrated by al-Tabarani (11763) in al-Mu’jam al-Kabir and Ibnu Hibban (4116)
 See Tafsir al-Maraghi, 4/217