You and your brother are of the same age, so the children in your brother's family are a generation younger than you. Are you him (her)? Uncle, he should call you uncle, and you should call him nephew (female)

But since he is older than you, by rights, you should call him by his first name instead of his nephew (don't call him by his first name and last name, remove his last name, just call him by his first name, which will be more intimate).