Following up on my previous comment, and Mahiro's comment three pages back on how his current body fits the situation better, look at his height. He's now shorter than his "little" sister. From an outside perspective, here it looks more like he's become the younger sister (which, in a way, he is!), while his sister has assumed the role of the eldest sibling. Those tropes are definitely being invoked deliberately; the less skilled sibling is now in the role of the younger sibling, while the most skilled and talented one is now in the "proper" role. They're even acting in those roles now. The "eldest" is looking after the "youngest".