You can't claim your sibling as a dependent unless your parents either:
a. have such a low income that they don't have to file a tax return. If they do file a tax return, they must only request a refund of withholding and can't claim extra deductions, credits or dependents,
or
b. your income is more than your parents, and they agree to let you claim your sibling instead of them. If your parents want to claim your sibling (their child) as a dependent, they have the absolute right to do so, even if you make more money than your parents and provide more support to your sibling.
