So I have a question about the rules

You can put it in your steam and give it to your brother.
next time , check the game before you enter to win him, espically if you are a mac user.

your brother can play on your acc -_-

I guess the right thing would have been to thank the creator of the giveaway and decline the gift and tell the creator to submit a ticket and ask for a re-roll of the giveaway.
So, basicly, you should not have accepted the gift in the first place.
Now that you have won you have to register it to your Steam account or you risk a ban.

You can use bootcamp. You can wait to play it until you get a proper pc. Or if you really mind these options and don't want it rotting in your library then you can ask the creator of the giveaway to ask for a reroll so that someone else can get it.

Hey there, you can actually run Windows on your Mac or get a new PC.

Or you can create support ticket to re-roll this giveaway.

Ask support for a re-roll.

Wait so do I have to contact support for a re roll or ask the giveaway creator?

I couldn't help it but I looked to see what you had won and it was Binding of Isaac. Last time I checked that game is available on both the PC and Mac, or if there is something i'm missing you could always run bootcamp (or wine depending on your hardware) if you really want to play it.

Submit a support ticket explaining this and let the giveaway creator know so he can re-roll.