How do I convince my brother to always fuck me instead of his girlfriend?

I (13f) and my brother (coming up on 15m) have been fucking for a while now. I know someone in my life is going to be very disappointed in me if I keep pretending like we haven't been doing it all this time, but I'd rather be completely open about this than keep it all locked up inside going crazy. It feels amazing, so natural, so right. I love that he's still young. No one ever has to worry about me getting pregnant, since we obviously use condoms.<br>One of the girls in my class has a crush on him, and I know he's talked to her briefly about going to the movies with him. I feel awful about how jealous I am every time I see her around, and I know he likes her too. This is really the only thing I can’t stand about our relationship.<br><br>I want to tell him about how much I love him and how jealous I am that that girl has been flirting with him. I want to tell him about how jealous I am whenever I hear him talk to her. I know he doesn't love her, but I have to admit I’m a little envious she’s been where I’ve been. I think I’m going to cry if she ever screws him. I want to know how I can stop myself from crying so much about it.<br><br>What do I do?