When Kiku said "I love you" i hope she means it like just her not because Sena is her ex sister. More than that when she said "I want to become your lover" it didn't sound like in romantic way, i had a feeling like she wants to say "I can be your sister replacement(you can do anything you want with me) " , i would like it if she said "I want to date you" or ask "Will you go out with me?". It's like they still can't let go of the feeling for the sister(Sena the most).
Sena is the most broken character here.
Kiku, well I like her character but on the other hand i would like to see her point of view, I want to really know what she meant when she ased Yuuna if she can have Sena and what was Yuuna response because I don't belive she end the call after she heard that. Also the more I read it the more I wondering if whether they really dated, besides for how long they were together, because as you can see they broke up in friendship and have a good relationship, so a long-distance relationship could be successful for them. More and more I am convinced that Kiku was in love with Sena from the beginning, and Yuuna, knowing this and knowing that she would go abroad, decided to introduce Kiku as her girlfriend, so that Sena and Kiku could become friends and when she was gone Kiku could keep an eye on on Sena and thus get closer to her.