Ngl, the childhood friend came in strong and could've had the upper hand, but lost with that last sentence.
She could've just went with the route that she can help it become better, since obviously she can guide him better being a well know mangaka, also train him and she also knows how to interact socially, leaving nothing for the little sis to argue about. While also doing whatever she wanted to do, secretly.
OR
This was all her plan to improve the siblings relationship and actually make ShinKano more successful by acting as a rival and getting both of theirs competitive spirit up.