How to get minecraft on a laptop with Windows 8?

So I downloaded the game and I wanted to play multiplayer with my girlfriend, but it just said server cant connect but the internet was working and everything. My sister has paid for the game, but I want it on my laptop too but I have to pay for it again besides the fact my family have already paid for it. I just need multiplayer, that s all. Ive also tried all the free minecraft downloads but they don t work. I just need to download the game with multiplayer, can anyone help?

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