Artist is clearly capable of drawing women looking mature so I don't know why they don't for Lurine. She's immature and childish and on top of that she's short as hell. There are plenty of women who are short and can be childish and still... Portray and carry themselves as an adult. Literally when the romance between starts, the people around her comment on her growing and her response is, "is it because my coming of age ceremony is coming up?"
Fiction is fiction and nothing can change that, and her existence really does not care in the grand scheme of things, but people are 100% valid in feeling uncomfortable with the pairing considering how she was portrayed up to this point. It literally all comes down to poor character and In general story writing.
In the end, the characters feel so cartoonish and cliche compared to the main character which just makes it feel much more corny and embarrassing. Maybe if she felt more 'real' people would be less uncomfortable, but unfortunately her general image is "younger bratty sister" and there's not really much you can do about that.

Only the internet can see a grown ass woman and call it pedophilia fr