Kardashian posted a photo of Jenner spliced side-by-by with Baddiewinkle, an elderly social media personality known for her modern fashion sense.
"Sorry Kylie, Baddie looks better in @fashionnova 😂😂," he captioned the photo.
Jenner posted her original photo, which features the cosmetics entrepreneur in a gray knit top and pants, on March 7 as an ad for Fashion Nova.
However, Baddiewinkle was actually seen in the outfit first on March 5 with a post captioned "stealing your man in my @fashionnova 🌹."
The Keeping Up With the Kardashians star's followers are loving the jab directed at his half-sister, as the photo has more than 114,000 likes.
This isn't the first time Rob has taken to Instagram to poke fun at his siblings.
Back in September, he posted a collage featuring the famous family (including himself) photoshopped with Kim's newly blonde hair.
In the past, Rob and Kylie have had more than a few issues with one another.
Their most infamous conflict came when he leaked Kylie's phone number to his Twitter followers.
The conflict stemmed from a baby shower disagreement back when Rob's then-fiancee Blac Chyna was pregnant with their daughter Dream. Kylie wanted to throw a baby shower for Rob, but didn't want Chyna to attend. That didn't sit well with Rob, so he posted the number during a barrage of tweets about the incident.
The two have allegedly made up since the incident, so this latest post was presumably made in honest jest.
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