This may be the one time I didn't think the exchange between the sisters was fake. I think Kourtney is just lost, Growing up, I think Kris felt that Kim was the "pretty one," so Kris favored her. Then Kim became exponentially more famous and successful and the favoritism continued. The one thing Kourtney had for a while that Kim didn't have were children; so I think being a mom became Kourtney's "thing". Now that they both have children, Kourtney tries to distinguish herself from Kim by claiming (rightfully or not) to be more of a hands on parent. Kim calling Kourtney out, claiming that the reason Kourtney didn't want to do shoots etc. was because she was lazy, rather than because she was some super mom, and then reminding Kourtney that she'll never be as exciting (physically or fame-wise) as Kim probably fed into all Kourtney's insecurities/struck a nerve-- so she just lost it.