I am not your mother, but I am your big sister.
I cannot even apologize for it, I am always going to act like your second mom. I am going to keep yelling at you to (please) put down the toilet seat and to clean up the mess you made in the kitchen. It doesn't matter to me how often you say "I am not your mother," because you're my little brother and I'm always going to be the boss.
I never mean it when I tell you to grow up.
I hope that you have taken, and continue to take, full advantage of your childhood. As often as I complain about your maturity level, my wish for you is to put off growing up for as long as possible. The closer I get to real adult life, the more I miss home and all of the worries I didn't have. You shouldn't rush through the years you have left at home, you are doing just fine the way you are.
No, I didn't tell Mom.
All of our secrets will always stay secrets. I may have ratted you out to Mom about being the one to break her new vase, but I hope you know that our brother-sister bond protects all of the private things we share. Please, never forget that I'll always be here to listen to you.
I'm sorry for giving you your first bloody nose, and for laughing at you afterward. I'm sorry for every time I have blown you off for plans with a guy, or to get an extra hour of sleep. I'm sorry for yelling at you to leave me alone and for slamming the door in your face. I'm sorry for all of the times you asked me to play outside that I didn't. I'm sorry for all of my broken promises.
I forgive you.
I forgive you for all of the “little brother" insults you have used. I forgive you for using all of my paints and letting them dry out. I forgive you for embarrassing me in front of every guy I ever brought home. I even forgive you for cutting off that piece of my hair in fourth grade.
I am so proud of you.
It isn't said nearly enough, but I am so proud of you, little brother. I am envious of the passions that you have and the way that you pursue them with no fear! I am excited to see where you go in life (but don't go anywhere too quickly). Keep working hard and doing what you love, no one can fault you for following your heart. I love you so much, and I will always be your biggest supporter and fan!