بسم الله الرحمن الرحيم
(Fatwa: 353/326/M=3/1438)
If the relative who is going to marry is poor and eligible to receive zakah and you hand over zakah to him and give it into his complete possession then your zakah shall be paid. Further, if that poor relative invites you in marriage party, you may attend it and eat therein. The ruling changes when the ownership changes. But it is not correct to feed with the zakah money directly without giving it into his complete possession. If that relative is not eligible to receive zakah, then it is not lawful to give him zakah.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband