It's just that you see them as family while they dont
Well technically its 2 different situations since those are you blood related siblings while gerald was adopted meaning he shares nothing about the family except the last name but I can understand what you mean because if I was raised with someone who is like a sibling I wouldnt see them any other way
That is why I said .... familycest
Its uncomfortable #－.－)
I think the angle here is that he likes her (romantically) since they met, so he literally never, not once, thought of her as family but as a romantic interest.
From her perspective things are different, though. Can't say anything there, she really keeps thinking of him as a brother, but she suddenly has doki dokis for him? Seems strange.
I have a lot of step brothers, and they are family to me regardless of blood relation. I understand that he loves her, but its kinda cringey, like they are family she keeps calling him brother. Ugh