Considering the statement Arthur once made that Pal is lucky he "(doesn't) have a sister" in "The Chips Are Down", Pal's siblings might all be male. Alternatively, Arthur could have meant that Pal doesn't have a sister living with him, or, most likely, that he doesn't know the puppies' genders and is just assuming. In Sick as a Dog, Arthur tells his father that Pal has never been away from him. This could mean that Arthur adopted Pal straightaway after birth (which is illegal as puppies must be eight weeks old) or he could have adopted Pal later and they weren't separated since then.