This is definitely heading towards a poly end, and I'm completely fine with that.
Plus, I think there is some foreshadowing for it. Although, for reasons that have yet to be fully explained, Rinna has a strong sense of love towards Mei, she has also been shown to fall in love with others easily. And as for Sayo; she has been shown to have multiple arrows. Sure, they all point towards Mei, but just the fact that she has more than one could mean that she has the potential to love more than one person.