Something I noticed while rereading the chapter - if we go with the interpretation that Kiku fell in love with Sena due to that meeting on the bench, that means that the reason she and the sister broke up was likely because Kiku fell in love with Sena. At the very least, it happened before she saw her sleep-talking to the sister, since her expression and the heartbeat sfx imply she was already in love with her, added to the "I felt some kind of expectation" line as she was walking into the house.
This would make sense more than "Kiku and Yuuna broke up for unknown reasons and now Kiku is after Sena for nefarious reasons".