One febuary night i was out with a couple of my brothers friends, and me and one of his friends went to go buy drinks. We weren't on our 5 senses and we ended up having sex that night.. And up to this day we still have sex. Is it normal he is 21 and I'm 17? Is it bad?

You both want it, and I don't think the age difference is of importance.
Go for it ^^.
As long as it doesn't cause any trouble between your brother and his friend.
To the others: Well it hasn't caused any trouble with anybody but us, because of the age difference. And thanks for the Opinions(:
your brother doesn't own him. and hey if you get married at least you know he already likes your family =)