The 26-year-old reality TV star doesn't want to watch Kim, 33, and her fiancé Kanye West, 36, tie the knot in Paris, France, in May because he's unhappy with his appearance.
A source told RadarOnline: ''Rob is upset about his changing shape and truly feels like the black sheep of the family.''
The source added: ''Kim is his sister. He loves her. But he knows this will be all about Kim and as usual, Rob feels he would be like the third wheel.
''The reality of their situation is that Kim and Rob haven't been that close in years. Deep down, she thinks he is a total slacker and it's very hurtful to Rob.''
Meanwhile, Rob recently sought medical help on his weight and he was horrified after learning that he was at risk of developing diabetes and eventual organ failure.
However, the sock designer - whose father Robert Kardashian died aged just 59 from oesophageal cancer in 2003 - has agreed he needs to make changes to his lifestyle.
Superman is missing from the 'Justice League' trailer.
The 'Power Rangers' reminded Elizabeth Banks of that 'team' aesthetic.
Charlie Hunnam has described his odd relationship on the set of 'The Lost City Of Z' with Robert Pattinson, who he "didn't say more than 10 words to...
The two actors worked together on 2003 western 'The Missing'.
Captain America actor Chris Evans has hinted he'd be open to returning for more Marvel movies in the future despite his contract coming up.
They say you should never meet your heroes, but Michael Fassbender is glad he met Brendan Gleeson.
'Prison Break' returns in April for a fifth season, but how will Robert Knepper's character T-Bag fit into the folds of the new episodes?