Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
There is nothing wrong with marrying the daughter of your mother’s maternal uncle.
Answering your question, the Fatwa Center at Islam Q and A, states:
It is customary in some societies to call the mother’s maternal uncle “grandfather”, but this is not correct in either the Shar`i or linguistic terms.
Based on that, the daughter of your mother’s maternal uncle is regarded as your maternal cousin, and the (female) maternal cousin is not a mahram, so there is nothing wrong with marrying her. Allah, may He be exalted, says:
“O Prophet (Muhammad)! Verily, We have made lawful to you your wives, to whom you have paid their mahr (bridal‑money given by the husband to his wife at the time of marriage), and those (slaves) whom your right hand possesses — whom Allah has given to you, and the daughters of your paternal uncles and the daughters of your ‘Ammaat (paternal aunts) and the daughters of your Khaal maternal uncles and the daughters of your maternal aunts” (Al-Ahzab 33:50)
Sheikh `Abd Al-Rahman Al-Sa`di (may Allah have mercy on him) said:
Allah, may He be exalted, said, telling His Messenger of that which was permissible for him (for marriage) as well as for the believers, “and the daughters of your paternal uncles and the daughters of your paternal aunts and the daughters of maternal uncles and the daughters of your maternal aunts”. This includes (daughters of) paternal uncles and aunts, maternal uncles and aunts, near and far.
Allah Almighty knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.