I slept with my best friend, then with his brother. what do i do?

The fact is, your friend asked you not to, you said you wouldn't, and then you did. Everyone makes mistakes, but this is something you can learn from for the future. Next time, issue some self-control and don't let anything 'kind of happen' if it isn't what you really want. But every action has a concequence, and the consequence of this may be a lost friendship. Blood is thicker than water - your friend will be angry with him, rightly, but his family will still always come first. He isn't going to want to go where his brother's been, and the sad truth of it is, probably a large amount of respect he had for you has been lost. It won't be easy to get this back. He is unlikely to change his opinion of you, at least anytime soon. Your best shot would be to give him some time and space to accept what happened.