Welcome to AS Melissa,
Does your Mom know that something happened? If she does, why haven't you told her who did it?
You've got to remember, she was older, she did it, you didn't know any better. And even if you thought it might be wrong, she told you it would be fun. Don't blame yourself! We've all done things we're not proud of, but this isn't something for you to have on your concience, it's her problem. You need to heal now.
Please take care of yourself and feel free to talk here, everyone's really supportive and understanding.
Michelle