Khloe Kardashian hosted nephew Mason Disick's first sleepover Sunday evening.

The 31-year-old reality star proudly announced the news on Twitter the next morning, writing, "Mason had his first sleepover at my house. He had the best time with [my pet dog] Gabbana and I! It was harder on Kourt. She's the cutest mom."

At age five, Mason is Kardashian's oldest niece or nephew, and the eldest child of Kourtney Kardashian and Scott Disick. The former couple are also parents to three-year-old Penelope and 8-month-old Reign, while Kim Kardashian and Kanye West have two-year-old North.

Kim Kardashian jokingly argued with her younger sister over Mason's milestone, tweeting, "Ummmm his first sleepover was with me in NYC... Nice try." The 34-year-old television personality and West revealed they are expecting their second child, a boy, in June.

Kardashian and husband Lamar Odom struggled to conceive during their four-year marriage. The reality star filed for divorce in December 2013, and the former couple signed to finalize the procedure over a year-and-a-half later in July.