Molly and Rover are Snoopy's brother and sister. They appear in the TV special, Snoopy's Reunion. They apparently exist in the universe of the Peanuts comic strip, as shown when Snoopy's father receives a letter from all his children and mentions that all eight of them have signed it. However, unlike the rest of Snoopy's siblings, they never appeared and were never named in the strip. They most likely would have appeared if Schulz had been able to keep writing the Peanuts comic strip longer.
Molly appears to look very similar to her sister Belle, except that she wears a blue dress. similar to the one worn by Lucy. In the special, she plays the mandolin. Molly is shown to be rather pampered because she has her own make-up. Rover looks like Snoopy, except his long floppy ears are brown instead of black. He plays the dobro.
Both these siblings grew up on the Daisy Hill Puppy Farm, and were eventually sold to different owners. Like all the siblings, they are shown to be upset when they find out that the Daisy Hill Puppy Farm had been demolished and turned into a parking garage. But in the spite of that, Snoopy, Molly, Rover, and all the puppies are happy to be reunited and play more music. They play one last song before returning back to Charlie Brown's house to end the reunion.
In the December 18, 1998 comic strip, Rerun van Pelt wants a dog for Christmas and wants to name him Rover. This is possibly a reference to Snoopy's brother.