It came back with the little sister, and that just lead into the "monogamous" relationship she is currently tip toeing in currently.
If it came to light that the little sister was behind that encounter, and all the dirty laundry comes out, it could be a good plot. We have a dozen girls after 1 person, and she's dating a yandere with a list. Culling will happen regardless
Oh certainly, I think Arare is going to eventually slip up and reveal how much of Itsumi's isolation was orchestrated by her. If she also mentions how much that truly broke Itsumi, especially in conjunction with the Rin's unintentional actions doing similar at school, Rin might just make that kill book a reality.
I feel that Rin, and her friends, will probably be on that resort trip as thanks for coaching. Real question is if Arare catches wind of it and worms her way into being there to. At worst we have at least two arcs left, Summer Festival and Tropical Resort, but hopefully we have far more.