It's a shame the kittens will be separated, but I think both you and your brother should get additional kitties for companionship. Would you consider letting your brother take both kittens, and you can get another? It seems like these two are a bonded pair, and I suggest doing what's best for the kittens, even though it would be a very tough decision on your part.
I agree with Dearpie. Think more of the kittens, and let them stay together - either with you, or with your brother.
One of my cats lost her "brother" when she was younger, and she has
never bonded the same way with another cat. It is really sad, honestly.
