Where did you get that from? Cuz the synopsis says none of that. Tons of games have different protagonists within the same franchise, tied by themes and gameplay. I'm pretty sure they won't establish in-game if the protagonist is the same or not. He's just a silhouette with no features anyway, mostly a self-insert conduit. He can be whatever you want him to.
Also, them being Hazuki's sisters makes no sense, they live on a different home, and from the description, the younger one is just like Hazuki anyway. If they were to be related, they could just use Hazuki + a new girl, no reason to make a copycat. They look similar because Summer sold a lot, so they're sticking with body types that worked.