man, probably it wasn't the intention of the writer, but i think souta talks about her sister because he just doesn't have a life of his own, those perverted girls want to attribute it to him being in love with his sister, but he probably thinks he just doesn't have anything else. how could he? they use him for every mundane chore they don't want to do, what is he going to talk about? cooking for everyone? doing laundry? doing groceries? he probably thinks nothing of what he does is nearly as interesting as his sister