my life is about to get complicated.

Let your brother have sex with your girlfri- oh wait.

Surprised nobody has called rape yet.

But that aside, You should tell him.
As a previous person said; You wouldn't want to be with somebody who cheated on you, so neither would he. Let him know what happened. Regardless of her being 'drunk out of her mind', Unless the two of you look fairly similar, I'd say it's likely to happen to anybody. She is responsible, as are you. Tell him, let him know what he's dealing with. You're family, he may not be as harsh with you, but don't expect to get out of it lightly. You still fucked over your family, which is rude. The longer you leave it, the worse it'll get. He WILL find out. She'll tell somebody, they'll tell somebody, it'll get back to his friends, who'll mention it to him.


tl;dr: Tell him. It'll save him a long relationship of heartbreak, and the consequences won't be anywhere near as severe as if you'd left it a long time, and he found out the hard way.

Either way, you're fucked. If you leave it, he'll find out when she's drunk, or eventually, and it'll be worse for you. It's better off coming out of your mouth than out of hers or one of his friends.