"North acts like an only child at all times," she told host Ashley Graham. "She's beyond jealousy now."
Kardashian is parent to North, 2-year-old Saint and 8-month-old daughter Chicago West with her husband, rapper and designer Kanye West.
She said North copes with Saint by keeping him at arm's length.
"She said to me the other day, 'Mom, I have a great plan. We just need to have another baby brother so Saint can just leave me alone. So the girls can be on this side of the house and the boys can be on that side of the house,'" the star shared.
"She has a sign on her door that says 'No Boys Allowed,'" she added.
"She won't even let Saint step foot in her room. If he does, or if he's with me and I have to take care of both of them, or get both ready, it's like full war. It's crazy."
FOLLOW REALITY TV WORLD ON THE ALL-NEW GOOGLE NEWS!
Reality TV World is now available on the all-new Google News app and website. Click here to visit our Google News page, and then click FOLLOW to add us as a news source!
In contrast, Kardashian has previously said Saint and Chicago are "inseparable."
She told People in July Saint is "obsessed" with his younger sister and also loves his cousins, including Stormi Webster and True Thompson.
"Saint, every time he sees Stormi and True, he's like, 'I have a baby! Do you want to see my baby?!' He is so obsessed with having a little sister. It is really, really cute," the star said.
"They are all so good together, but I think Saint is probably most excited about all the babies," she added.
Copyright 2018 United Press International, Inc. (UPI). Any reproduction, republication, redistribution and/or modification of any UPI content is expressly prohibited without UPI's prior written consent.