I'm planning to go behind my brother's back to give my nephews Santa gifts.
Why would you give gifts behind his back?
I don't like that he doesn't want them to have gifts.
I understand why you want to give your nephews gifts, but if your brother doesn't want them to have any then you shouldn't give them any. If you want to give them gifts I think you should talk to your brother about it first.
It's okay, I can handle myself.
I think your brother has a reason to withhold gifts and you should not overstep. Have you talked to him about why he wants it this way?