بسم الله الرحمن الرحيم
(Fatwa: 968/968/M=09/1436)
If your married sister is poor and eligible for zakah then you may give her zakah. It is not necessary to tell her that it is zakah, just your intention is enough and your zakah shall be paid. However, do not give her such a huge amount at one time which turns her sahib nisab rather give her the zakah money in fractions as per her requirement.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
