@twinklecake That is a really good question.
I think a nice way of seeing it would be to now the underlying motivations and reactions of those involved; Nazuna and Kou have a healthy relationship, they bicker and play but care for each other; Hatsuka situation is strange, by any means their interactions with those three "friends" is strange, but at the same time they seem to truly care for them it is an overpowering relationship but it is functional and those involved are satisfied; Kiku and Mahiru situation just doesn't click well, because to Kiku this seems like a game, she doesn't care for Mahiru or any of her "servants", they are a challenge only to attractive to the point were the give away to her, be it with her conscious intention or not, and by what she says she maintains them close just out of habit.
This is how i see things, this story isnt really reinventing the mold but its execution about the nature of relationships is really well made.
Edit: Just a little observation... i have the impression that the trio in Hatsuka's home arent vampires, if i am right this sheds a different light on their relationship.