Firstly, i think you should not call your cousin whore. After all she is your cousin, and you are the person she cares. You shouldn't talk about her bad with others.

But i know that in fact you hope your cousin becomes better, not like bitching.You should tell her your inner idea.And also you should talk with your friends and your cousin's boyfriend.

I think you just need to sit down with your cousin and have a long talk with her. Either you guys go your seprate ways and make up. Then you need to do the same with your friend and her boyfriend. I think this is just a problem that has been created by keeping too much information and opinions bottled up, and now is the time to constructivly let it out.

I know it's going to be hard for you to do, but honestly telling your cousin what happened and why you did it is the best way out of this. Perhaps say something like, "I called you a whore because of some of the things you did...I'm sorry, I know I shouldn't have judged you like that. I hope you can forgive me." Let her know, you know it was wrong of you to say that, and how you feel about it.

Then, after that is done, telling your friend and her boyfriend you're sorry and that you came clean with your cousin and apologize to them is the next step. Again, just admit what you did was wrong. You might just gain their respect for it, because it is a very hard thing to do.

In this situation, the best thing to do is just be honest with everyone, and own up to your mistakes. There's no other option that I see. You messed up, now you just have to clean it up. And it's okay that you messed up. Everyone does. Things just got out of hand a little, which happens a lot, and now you just have to say your sorrys, hope they forgive and try to move on whether they do forgive you or not.