Your bother can only be listed as a dependent on one tax return. If your father is qualified to claim your brother as a dependent, then you would not qualify to claim him since 1 of the requirements to claim a sibling as a dependent is that ''They're not claimed by anyone else''.
If for some reason your father is not eligible to claim him, but he claimed him anyway, even though you are eligible, you can still list him as a dependent on your return. However, the IRS won't allow both of you to claim your brother. Dad would need to remove him if this is the case.
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Your return will be rejected. You can file by mail with an explanation that you are aware that your brother has been claimed and why you should be able to claim him. The IRS will sort it out from there. If the father agrees that you should be able to claim him, he should amend his return.