Personally *I* think Ralph & Kirby are brothers so... this is how this picture spawned.
The other two dogs are *obviously* their parents. Their mother was a sled dog and their father was a house pet. Their parents were old when they gave birth to Ralph and Kirby. Their father was nearly ten and their mother was eight. They lived through the epidemic but died shortly afterwards.
Ralph & Kirby (C) Universal
Their parents (C) Me