A source told the gossip site that McNearney had organized the whole stunt, in which Sidibe took her place down the aisle.
"Everyone was standing up when it was time for Molly to walk down the aisle, but instead of Molly, Gabourey appeared and took Molly's walk down the aisle," the source said.
"It was so funny! Jimmy was 100 percent completely surprised -- he had no idea. Molly resumed her walk down the aisle after the joke ran its course. She even set the whole thing up!"
Kimmel's younger brother addressed the prank in his speech.
"I am John, brother of Jimmy and brother-in-law of Gaboury Sidibe," he quipped.
McNearney is head writer of "Jimmy Kimmel Live!," so she knows how to write a joke. The couple got engaged at South Africa's Kruger National Park in 2012.
Sidibe, 30, has participated in several of Kimmel's popular video sketches. Most recently, she appeared in "(I Wanna) Channing All Over Your Tatum," along with Miley Cyrus, Matt Damon, Olivia Munn and Jamie Foxx. [GIF courtesy of Crushable]
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