cinderella, your option is to talk to your parents and let them know what you know. If the only thing you are concerned about is your own reputation in all of this, then you have nothing to worry about. If you are concerned about your friend’s reputation, then you should speak to her about your concerns. If you are concerned about your brother’s reputation then speak to him about your concerns. That is all you can do.
You can’t control other people just yourself. You can’t micro manage what other people do. Everyone is responsible for their own choices. You can only speak to them about your concerns and offer them guidance, but you can’t force them to see things your way or to fear what you fear. It seems that your fear is more about what will happen to you in all of this and that is not something that you should be afraid of. Concentrate on being the best you can be. That is really all you can do. Lead by example and concentrate on who you are and who you want to be. Don’t get caught up in their business, their business will only effect you if you are part of it.