Do I have to download Minecraft twice for one computer?

Solution 1:

No, you do not have to download another copy of minecraft. Even if you purchase a second account, you still would not download a second copy of minecraft.

If you are planning on playing single player(offline) then you can just create two separate worlds. One that is yours, one that is your sisters. You don't need two accounts to do that. You can make several worlds if you want.

If you want to play Multiplayer(online) on a server, you can still share an account. I would recommend you both play on different servers. Though, if you wanted to, you could both use the same account and play in different areas of that server.

A second account would allow you to both log into the same server and play your own games with your own game name.

Solution 2:

I might be misunderstanding your question but, when you buy Minecraft you get one Minecraft account. You only need to download the game once and then you can access both accounts on Minecraft.

As far as I know, there is no way to download minecraft twice.