Answer
(Fatwa: 2165/1992=D/1429)
The son of brother-in-law is the girl’s cousin brother (son of maternal uncle), if there is no legal prohibition, a girl can marry her cousin brother. Those deeming it incorrect are wrong, it is their perversion in belief that the Shariah has made something lawful but they regard it unlawful and wrong. So, they should be convinced that the act of theirs is wrong and they will be accountable before Allah for that. In order to break this bad custom, if the mentioned person gets his daughter married with her maternal cousin brother, it will be right. Also, he will get reward of breaking this evil custom. But, it is better to persuade them first with politeness and wisdom so that there is no ill-will between them.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband