As a child:
"Brother, you are mine."
"Brother, you have to show me before you get a girlfriend from now on."
"Brother, I like you the most!"
After growing up:
"Brother, you can only be mine, and I am also yours."
"Brother, you can only stay with me for the rest of your life. Don't even think about having a girlfriend, and you can't even have a boyfriend!"
"Brother, I still love you the most."