I'm going to go behind my brother's back to give my nephews Christmas gifts.
Your brother has no idea you're planning this?
He doesn't know anything about it.
I think that you should run that by him first though. You should not go against what the family had already had planned.
Why not? Do you think it's a bad thing?
If your brother doesn't want his children to receive Christmas presents, then you should respect his wishes. If he has no problem with them getting Christmas presents and you are doing it just as a surprise for them then it's perfectly fine. Just make sure you aren't overstepping your boundaries and doing something that your brother doesn't want.