Former Threshold actor Jake Abel has joined the cast as Adam Milligan, an 18-year-old who is revealed to be Sam and Dean's younger sibling.
Speaking to TV Guide, Kripke admitted that he was aware "fans would scream that we had jumped the shark", a phrase used to describe absurd storyline departures.
"Of course, adding a new sibling or relative to an established series is a classic and cherished shark jump," he explained. "For fans who are worried... watch the episode, decide for yourself. Personally, I don't think we're gassing up the motorcycle quite yet, but you tell me."
Joking about his decision to name the episode 'Jump The Shark', he said: "It became clear that we should beat the fans to the punch... though rejected titles included 'When Leo Joined Growing Pains', or 'The Raven Symone Episode.'
"And stay tuned for next season, when the boys take trips to London and Hawaii and Sam and Ruby get married. And Ted McGinley joins the cast as the thoughtful college professor!"
'Jump The Shark' will air in the US in April.