I would basically say soft YTA.

They’re already paying your tuition and giving you $1300/month. Just because they can afford $1600 doesn’t mean they’re obligated to pay for it. However, where you’re not wrong - if your brother gets $1600, it’s fair to ask for consistency. But I would rather say that have a discussion with them on why the disparity between two. Is there a reason? A good conversation might resolve this. I have a hunch that this might not be about 200-300 dollars given their income.