As an addendum to previous replies, if per the IRS rules you are legally entitled to claim your younger brother, then by all means do so. Just be prepared to prove your right to claim him, because if anyone else claims him also then both of you "will" get a letter from the IRS requiring you to substantiate (prove) your right to claim him. The one who proves it, gets to retain the claim. The other party will be required to pay back any tax incentives received for falsely claiming him, along with interest, and may possibly be assessed a fine too.
