There's also the shock because it's so sudden; and romcom effect make them untouched by other men lest readers march brandishing pitchfork to the publisher's office, so they don't have enough experience of just being casually close to male. It could just be them starting to explore if their feeling of closeness to Arata "is actually love but I just didn't realize it until now" or simply being family.
I still hope at least half of them get over it and decided they really just want to be family though. And it would be nice if they could explore similar feeling with Shion, although that guy got a shield (girlfriend) so they would have seen him as off-limit.
Although... again, readers would be angry if the girl then fell in love to another guy not the protagonist.