We've all had our crushes on people who may be older than us, and at the tender age of 13, it is important that you respect her feelings. But it also important that you not lead her on. That being said, here is what I think you should do. Either call her/visit her or wait until she decides to pay you an unannounced visit, and explain to her that though she is a nice person, it would be ILLEGAL and morally wrong for you to date her, but you would love to have her as a friend. I'm sure she'll refuse, so tell her that if she persists, you will have no choice but to PERMANENTLY end your friendship, and ask her if that's what she wants. Tell her it's her choice, and you hope she makes the best decision so you can still be friends. If that approach doesn't work, maybe you could tell her that there is an "unwritten" rule that states that a guy cannot date someone related to his cousin's girlfriend b/c if they were to get married and his cousin married her sister than it would be weird. That answer is a little more complicated and less clear cut, but you get my idea. Just make sure she understands that she can either have you as a friend or not have you at all. I hope it works!!