help me tell my evil older sister something...?

You're wrong.

You can't handle yourself. You're making a mistake. Of course its bad to sleep with someone 9 years older than you at your age. In fact, its not just a bad idea, its really obviously a bad idea. Any guy who has to sleep with high school girls 9 years his younger is going to be a mal adjusted individual.

If you don't want anyone telling you this, you definitely shouldn't tell your sister at all. I hope you do tell her though, and I hope she tells your parents, and they ground you so you never see the night sky until you leave for college.

Well when you say hooking up i have to assume you two aren't really in a relationship. Id just tell her you met a guy (dont mention the age first) explain why you like him and all the positive things. If she can accept that the proceed to tell her the age difference. No matter how you say it she might be pissed off a little to begin with, but shell get used to it :) good luck

Why are you sharing everything with your sister if she's evil??

Your Sister will obviously not be too pleased, but it is your life.

Perhaps you should wait a bit just in case it does not work out. Doing so will avoid the "TOLD YOU SO" comment.