Praise be to Allah, the Lord of the Worlds.
If you are paying from the Zakah {obligatory charity} money, then you may give your nephew part of it if he is poor or indebted due to borrowing money to cover marriage expenses.
If what you are paying is from the voluntary charities and donations, then you may give him whatever amount you want and Allah the Exalted Will Reward you for that. This is based on the following narration: Salman bin Amir Dabbi narrated that: the Messenger of Allah said: “Charity given to the poor is charity, and that given to a relative is two things: charity and upholding the ties of kinship.” {At-Tirmithi}. And Allah The Almighty Knows Best.