بسم الله الرحمن الرحيم
If your sister is not sahib-e-nisab, i.e. she does not own cash equal to the cost of 52.5 tola silver or the nisab of gold or silver or some gold and silver with some cash money whose price reaches to the nisab then in such a case you can give zakah to your sister. And for the payment of zakah it is not mandatory to mention that it is zakah amount, you can give her without telling her.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.