I fooled around with my best friend's brother in her own house while she was there. and she doesn't know...
Do you think she would be angry if she found out? Is that why you're keeping it a secret?
I think she'll be mad and won't speak to me again.
Keeping a secret like that is a bad idea. While she may be upset, you are still probably better off being honest about what happened and apologizing.
I'm going to come clean with her soon. I feel really bad.
Coming clean is a great idea! You'll feel less guilty about this once it's out in the open. Hopefully you guys will be able to move on and stay friends.