But it's happened a lot earlier than anybody expected!
North, 23 months, and two-year-old Penelope were spotted getting into a little cat fight after ballet class last week.
During the short clip, North appears to start the 'fight' by flapping her arms in Penelope's face. But P soon hits back, grabbing onto North's custom made Balmain jacket.
We're sure their adorable disagreement didn't last long and the two cousins were soon back to talking about important little people things.
Penelope's mother Kourtney, 35, recently opened up about her bond with North, insisting the two girls are already close.
She told Harper's Bazaar: "Both of the girls love shoes – Last night North was over and they were fighting over shoes in Penelope's closet and they were so cute.
"By the end they were sharing and trying the shoes on each other and having so much fun. It reminds me of how Kim and I were when we were little girls."