So Yuuna did know about Sena feelings. Maybe Kiku was her 1st escape route (that flashback). Heck, maybe it wasnt even unrequited to begin with. All lies on what she'll say next. If was a guess game maybe Yuuna tried to run away using Kiku failed and later actually fall in love for her current fiancé? After all it would be really sad if Yuuna is still getting married to run away from her sisters love.
But nonetheless idk how will this affect Sena tbh. In a sense, was easier for her to let go with her sister away. Now it kind of lit the fire all over again.