Hi OlamHaba.
My two cents would be to the child there are many ways to serve Hashem and to do what Hashem wants us to do, and even though you are working on the kind of music you listen to, and doing a great job, your brother has not gotten to that yet. But there may also be things your brother is working on that you have not gotten to yet, either.
In this way the child can get the idea of individual growth and development and be rewarded for his success, but not at the expense of him thinking his brother is bad…or something along those lines.