What do we know about Jupe's family?
Robert Arthur never delved into the history of Jupe's family in the stories that he wrote. At some point he may have planned to do so, but I tend to think that he intentionally left that somewhat vague. We know that Jupe was orphaned at a young age and that (apparently) Titus Jones was Jupiter's father's brother. Of course, it's possible that Mathilda was a sister of one of Jupe's parents, and he changed his last name when Titus and Mathilda adopted him. Jupiter's acting career probably occurred before his parents died because it's hard to imagine Titus or Mathilda encouraging such a venture into the commercial world.
Marc Brandel added to the life of Jupiter Jones in "The Mystery of the Rogues' Reunion" when he described Jupiter's early life with his parents. He described Jupe's parents as being professional dancers. Brandel also described the parents' deaths in an accident, after which Mathilda Jones asked Jupe if he wanted to continue acting. Jupe replied that he didn't, and he went on to live with Titus and Mathilda.