We can't help having doubts about Kiku's feelings because the story hasn't given any reason to justify Kiku falling in love with Sena. Sena is a recluse with no friends and social life to speak of. Maybe Kiku is a secrete fan of Sena's books? It's hard to tell if Sena has even published any of her novels yet, let alone if Kiku is a fan of her work. At this point it's much more plausible that Kiku just wants to be with Sena because she looks like her sister.

this actually makes sense in a way, unless it is revealed later on what Kiku's true feelings are, this most likely can be an explanation of Kiku's feelings toward Sena