I have been an uncle since I was 5 and became a Great Uncle four years ago (I'm 31, my sister is 48 and a grandma). I am fascinated by uncles and nephews in the media, as many don't mention the mother or father of the nephew.
Kermit and Robin
Robin first appeared in Frog Prince in 1971 television special. Although Robin made a few appearances on The Muppet Show's first season, his relationship to Kermit was not established until Season 2. He was voiced by Jerry Nelson. He is now voiced by Matt Vogel. They were then translated into Muppet Babies, Robin was a tadpole.
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Here are some Uncle and Nieces or Aunts and Nieces
Scooby-Doo and Scrappy
Scooby-Doo was part of a ghost detective group of teenagers and he loved eating. Later on in the first series, his nephew was introduced. Scrappy was not a popular character but he was Scooby's nephew. Scrappy turned up in the live-action film as a villain.
Donald Duck and his nephews
Huey, Dewey and Louie were created by Ted Osborne and Al Taliaferro, and first appeared in a newspaper comic strip on October 17, 1937. They are identical triplets, they have also been given to Uncle Scrooge on DuckTales. 
Mickey Mouse and his nephews
His nephews aren't very well known but appear in a lot of merchandise and a few cartoons. Morty and Fredie Fieldmouse are their names. They started in Mickey's Nephews (published in 1932). 
Popeye and his nephews
Popeye's nephews Pipeye, Peepeye, Pupeye and Poopeye first starred in "Me Musical Nephews.” They are not as better known as other nephews in popular media._______________________
Here are some Uncle and Nieces or Aunts and Nieces
Inspector Gadget and Penny
Penny was the brains of the operation.
Minnie Mouse and her nieces
In the short Bowtoons, Minnie has twin nieces Millie and Melody.
Tenderheart and Wonderheart
In the current CGI series "Welcome to Care-A-Lot," the main bear Tenderheart now has a little niece known as Wonderheart.
Miss Piggy, Andy and Randy
Her nephews became more known in Muppets Tonight. 
