Both.My vote is that she is not joining the poly. After this chapter it feels she is being aimed at her own incest route because Mochi Au Lait.
Of the top of my head, she hasn’t explicitly said it, that doesn‘t mean she hasn’t shown it through her expressions and actions. Not to mention the difference in how she reacted to Yanagi kissing Toudou, and how she reacted when learning it was Rin whom Toudou was actually dating.Did Kurohime say she didn't like Yanagi? I only remembered Rin doing that.