
I skimmed the chapters to find it exactly, but maybe because I was so fast I didn't find the chapter where she talks about her age, either way she doesn't see either the prince nor the Duke as love interests because even if she finds them cute, she's mentally and psychically older.
And because Victor is more at her age, mature and caring, she falls in love with him.

People don’t care because they believe her to be his niece or daughter... why would they care? Also I’d think you guys were right if she didn’t make the parallel to her master having a sister and her being envious of that relationship. So her being all flustered was clearly about her being touched that someone treated her as family for the first time in her life; something she’s always wanted. As for her age, I don’t remember any of that and don’t think it’s particularly relevant, quite honestly.

But in the end, even if people mistake their relationship as sister/niece they aren't really related like that. People from that world (I mean the novel/comic) don't see them as family, and they don't really care if she's close to Victor.
Both her and Victor also don't see each other explicitly as familiar, only know Serenia is learning about things like that, so there's nothing wrong with wishing them to be together as a couple, as they would be so adorable together.

You’re missing the point. It’s not about how the people who are close to them would feel about it. It’s about public perception. It’s not because we don’t really see any of them interact with other people that it means no one would give a damn. They’re still nobles and public perception matters. Anyway, I really don’t need to argue over this more than I have because like I said, I believe her reaction had to do with being treated as family. They basically spelled that out for us.

This is just my last one too as you don't want to write anymore than that. I don't think the public'll really care, because they are capable of accepting her "dating" the Duke, even more after she put that young lady to shame in the start of the novel. Probably there would be a relief, as a stranger won't become duchess or even the crown princess.
Serenia in the end is not a noble from that world, Victor probably has a lower title or even none.
Serenia denying about having Victor in her heart was so adorable, I just waiting for when both of them confess