Your brother is a legal adult, and they can't stop him. You're not, and they can. Your parents are exercising the last bit of control they have over you. When you're 18, I'm sure they'll relax the rules a bit, but for now, they care enough about you to NOT leave you to sink or swim. If they didn't care, you'd have been left on your own a long time ago. Be glad they care at all.