Could you use oğlan kardeş or kadın kardeş?
(Also: Kim kardeşyan?) :-)
Lol at your joke :)
Nope, these are pretty much the only way to say (younger) brother and sister. :)
In daily language, "beş kardeş" means "slap". Generally used by worn out parents to warn their naughty children. (Beş kardeş geliyor!)