Unless your brother is a very young child who wants to give a present but doesn't have the understanding or funds to shop for or pay for it himself, he is massively cheeky.
I'd be inclined to play dim here and tell him you feel a bit bad that he's going to the effort of getting giftwrap and wrapping up the present when it should be you doing it, as it's from you. If he protests that it's actually from him, act very surprised and say "Oh! But I bought it myself and you haven't paid me for it, so how could it be from you?!"
Force him to spell out that he wanted the glory by pretending that your present was from him. Hopefully, he'll see his position is completely untenable and give you the money.
If he doesn't, wait until he's also present and ask your sister if she liked your present - say brother wanted to take it to her (maybe to ensure she got it ASAP if he was seeing her first).
The price is completely irrelevant. It's obviously significant enough to him to make it worth giving as a present (which it certainly is). If you were wealthy and he was skint, and he asked hesitantly if you would mind if he gave it to her as if from him - and he could make it up to you another time, then maybe. But just to help himself and steal your idea and effort, let alone your actual money that you paid for it, is outrageous. He needs to be called out on it if he doesn't stump up. This is absolutely not acceptable.
I don't know his marital/relationship situation, but that behaviour reminds me of the kind of Disney dad who treats their child, stops the money for it from what they pay their ex, meaning that she can't afford essentials for the children - and then keeps going on about how much more he loves them than their mum obviously does.