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.