As in, he can't buy a copy.
Can't, or won't? For example, if you are in the US, a lot of places are selling "MineCraft Gift Cards" (
https://minecraft.net/prepaid), and you can always have someone assisting you with getting a gift card such as a parent, older friend, and the like.
If he does not have the money, that I can understand. But I also remember being twelve, and begging everyone I could find to cut lawns, finding and saving bottles and cans, and pretty much doing the most absurd small tasks to get enough money to buy a GameBoy so I could play Pokémon. Of course, I didn't make more than forty dollars (in 90's money, that's, what, a billion now right? Hah) before Christmas/my birthday rolled around and my mother bought me one as well as Pokémon Red which had someone else's saved game on it (which I abused like nothing else, good times). But yes, I'm getting off topic...
So, is it something we can assist your brother with in regards to how to get a legal copy so you two can play together?