I agree. Early on he says that and a couple of panels later he's initiating sex. Based off of the chapters not too long after though, I think the indication is that he was lying and deluding himself about his feelings for her when he thought she was like his sister.
Yeah that would be the case. Cuz for most people the siblungs of ur best friends are like your too or at least thats how u start to treat them and protect them but feelings can grow later one so he jut said that to show he cares only bcuz it'll be wrong to think in a romantic way of ur friend sister or something its a guilt thingy most people feel and seeing how they might have some age gap probably not too much but ig at that era it would be a lot maybe idk he probably felt lile he shouldn't think anything of her and treat her just like his friend would oo
the only thing that bothers me is that Alexander keeps saying oh she's like a sister to me like that's weird stop saying that shit it feels creepy cuz why the fuck are you lusting after your sister then the art is so good had soooooo much potential but a little by this fact disappointed by this fact