Pal's siblings are the other golden retriever/English Setter puppies given birth to by Perky in "Arthur's Pet Business." It is unknown what happened to them after the episode, since only Perky and Pal appear later in the series. They likely were adopted or sold to a pet store, possibly Al's Pet Shop.
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 unsafe as puppies must be eight weeks old in real life) or he could have adopted Pal later and they weren't separated since then.
Pal is part English Setter on his father’s side, meaning his littermates would also be mutts.