Ok it is a bit weird that the first thing you wanted to draw of ur sibling is them nude but nude shouldn't be automatically equivalent to sexual. The reason why they drew the elder sister nude is cause then it represents how they see their "true" selves.
This was a really nice one shot abt siblings reconnecting with the younger reaching out first by drawing her older sister. Shes trying to tell her older sister that she sees HER and this leds to the older sister reaching back out and trying to reconnect with her younger sister, by telling her that no, this is who I am.
Her self portrait is more crude, less idealized because shes telling her younger sister to stop having this ultra-perfect vision of her. Shes trying to tell her that shes not as great as a person her sister thinks she is. And that she DOESN'T have to follow her in everything she does.
I do like this one-shot a lot, though i do think this story coulda also worked with non-siblings... perhaps fujimoto either is a bit incestual (... no comment) or really wanted to cover both the "true self" theme and the inevitability of being siblings?