Answer:
(Fatwa: 164/164=M/1429)
Islam has stressed that one should prefer religiousness while looking for match of marriages. So, you should not get your sister married to a boy who is certainly known to have been drinking wine. This only can guarantee the security of her iman and faith. Istikharah is undoubtedly Sunnah, but its outcome about the matter mentioned above is under consideration. If in future, you get positive signal from istikharah then also it is right to go against it.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
