Fiú means both son and boy, but there's a distinction in the
possessive case. Fiúm means my boy (boyfriend), but fiam means my son.
However, for girls, there is only one form: lányom means my daughter.
My girlfriend should be called by different words.
Siblings and cousins are mostly called with both gender and age
difference: "my younger niece" and not just "my niece".
Other cousins are created by adding másod-, harmad-, negyed- (i.e.
second, third, fourth) before unokatestvér, unokabáty etc., like
unokatestvér: cousin, having common grandparents
másodunokatestvér: cousin's child or second cousin
harmadunokatestvér: second cousin's child or third cousin