A nephew's inheritance
Fozzie's Uncle is, quite simply, the uncle to Fozzie Bear. He's mentioned twice in The Muppet Movie, where it's explained that he left his Studebaker to Fozzie. He hasn't died; he's just hibernating.
In a video promoting the Fathom Events screening of the film, Fozzie claims his uncle was still in hibernation as of 2019.