(1) If your uncle is entitled to receive zakah and you give your zakah into his complete possession then your zakah shall be paid. After taking it into his custody he may spend it for his daughter’s marriage, there is no harm.
(2) There is no need to pay zakah again, if your uncle is entitled to receive zakah.
(3) Give zakah amount into complete possession of your step-brothers and they can use it in their needs, your zakah shall be paid.
Allah knows Best!
Darul Uloom Deoband