Kim was replying to a fan who asked on MobioINsider how her daughter with Kanye West gets on with Kourtney's two children with Scott Disick. She added: "Penelope & North are bff!"
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While the two little girls are 'best friends forever', Mason – who's three – plays the role of the protective older brother.
"Mason bosses them around and is super protective," said 33-year-old Kim.
The little ones will have a chance to hang out over Christmas – as the whole family will be meeting up for the holidays.
When a fan asked who the reality TV beauty would be spending the festive season with, she answered: "Kanye, North, mom, sisters, step dad, brother, niece nephew, everyone!"
So the list encompassed her mum Kris Jenner, sisters Kourney, Khloé, Kylie and Kendall Jenner, brother Rob Kardashian, and stepdad Bruce Jenner.
"They are all my family and we will all be together!” the Keeping Up With the Kardashians star added. "I can’t wait."