Answer:
(Fatwa: 1356/202=TL/1429)
The sehra is a Hindu custom. The Hadith says: من تشبہ بقوم فھو منھم(one who adopts the ways of a community he becomes a member of it). Therefore, you should tell your inlaws that this is not an Islamic way; Muslims must refrain from such customs. If you convince them with wisdom they will hopefully accept it.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
