The brother of your father is also your father, as the sister of your mother is also your mother. But the sister of your father is your aunt and the brother of your mother is your uncle.
This also concerns the children. The son/daughter of the brother of your father isn't your cousin but your brother/sister, as the son/daughter of the sister of your mother. But the son/daughter of your aunt is your cousin, like the son/daughter of your uncle...