The mom of three, 36, posted a series of hilarious videos as she looked around her home for Kendall, 20.
“Hey Kyles, have you seen Kenny?” she asks Kylie, 18, who points down the hallway, noting, “I literally just saw her go over there.”
As Kardashian heads down the hall, she opens a bedroom door to find Kendall and Disick, 32, cuddling in bed together.
“Wait, who is she in here with?” Kardashian asks aloud, before she realizes that her ex has his arm around her younger sister. “What the …”
In the final video, Kardashian chases Disick out of the house, saying, “You’re so sick. You’re so inappropriate. Like, what is wrong with you?”
As he approaches his car, the father of three jokes, “Everybody knows I’m inappropriate!” Then, spinning around, he shouts, “April Fools!”
Earlier in the week, the exes also fooled fans on Snapchat when they joked that they were getting back together.
The pair, who dated for more than nine years, split in July 2015 after Disick was spotted cuddling up to ex-girlfriend Chloe Bartoli. They share kids Mason, 6, Penelope, 3, and Reign, 15 months.
Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!