My father step sister’s has a daughter & her son wants to marry my sister and I want to know is it possible or not in Islam.
بسم الله الرحمن الرحيم
(Fatwa: 992/988/B=10/1436)
Yes, this nikah is valid and lawful.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband