@chet31 not sure what you mean. You could be projecting or wanting to get some other meaning out of this. Sisters tone was cheerful/optimistic from the start. The gist was:
1. She's in love with the fantasy world that they're in and the opportunities to explore are infinite compared to what she had before.
2. "You had to give up on things because I'm with you. I don't want that. I'll take care of myself and my own matters. You shouldn't worry about me" In the other world big sis was basically fulfilling all the roles a mother needs to for her, which is a big burden to carry. She no longer needs to feel like she has to carry that burden , and can finally live life for herself as she hasn't let herself do before.
Also we don't see little sister at home that often. She's out in the world being the saint, whatever work that entails, which sounds dangerous. She's not dependent on her older sister for food/shelter aka flown from the nest. She's not a fully matured adult yet , she's still a curious wide-eyed spontaneous teenager, but one that has a job only she can do and people that rely on her. But when she's home, like when i'm home from college, I want to be spoiled to death by my mom and have home cooked meals. I don't think that means she's a spoiled child.
Granted, the visual tone of this chapter is very dark. A lot of things and people are shaded in a way that can be seen as omnious. But, shade from the sun is usually depicted as crosshatched. The little sister's front is shaded, so the sun is directly behind her. From the older sister's perspective, looking up at the sister is looking towards the sun which can symbolize the vibrance this world brought and can bring to their lives.