The 33-year-old star admits Mason, four, was furious about having a Coachella-themed birthday bash for 12-month-old North last month at his mother Kourtney's mansion in Calabasas, California.
Kim told PEOPLE.com: "When I said, 'North's having a birthday party at your house,' Mason said, 'No! I'm having a birthday party at my house! She can never have a party!' "
Kourtney, 35, added: "Before Kim even said we were having it at his house, he said, 'I'm going to put Legos everywhere! I'm going to scare everybody!' "
Kim and Kourtney's sister Khloé, 30, doesn't believe Mason is jealous of North, whose father is Kanye West, but thinks he takes after his "moody" mum.
She said: "He's very moody. He's very Kourtney-like. If he wants to do something he will. If he doesn't, you can't get him to do anything."
However, Kourtney is adamant Mason's temperament comes from his father Scott Disick, 31.
She added: "He might be a little more Scott's personality than mine. He's just the life of the party."
Meanwhile, Khloé insists Kourtney's two-year-old daughter Penelope is "exactly like" the brunette beauty, who is expecting her third child with Scott later this year.
Khloé said: "Penelope likes to observe and figure people out before warming up to them."