Dear Pathetic:
Your brother is going to end up a paving stone no matter what he does. He has no other options but that. What's your excuse?
If you're going to be so Good, why don't you try being honest on top of that? Tell the headmaster of your school that you're being distracted by compassion for your brother, who obviously isn't going to end up a wizard, and who's just acting as a punishment sink to prevent other students from needing to really try. Ask for your brother to be failed out of school now rather than later, so you're not being distracted anymore. You can always tell yourself that you'll come back for him later once you've struck it rich. It'll be a lie, but the part of yourself that engages in this idiocy will believe it.