I could tell you that the use of 'dog brother' came from author Robert E. Howard, creator of Conan, but it was used in his Red Sonya story titled 'Shadows of the Vulture'.
Red Sonja (spelling changed for the comic) was a character who appeared in the Conan The Barbarian comic book written by Roy Thomas. It is possible that a Conan issue which included her in it used that term for the first time in comics.
--Rafael--