I feel like as Sei comes to genuinely appreciate and care for Aya, it'll be too late. Aya will always feel she's second to Yoru and there's nothing Sei can do to change her mind, short of her outright repeatedly rejecting a buck naked Yoru who's begging for Sei's love with Aya witnessing each rejection.
The question is, will Aya in turn be the one who uses others to distract herself? Might not have been possible before but as we've seen, there's a darkness to Aya. Who knows what she would have done if Yoru hadn't stopped her. Is she going to cling to Haruki in an attempt to move on from Sei? Will Sei ever outgrow her obsession? She says herself she's "forgotten love" so maybe what Aya needs to do is make her feel what genuine love is supposed to feel like- though like I said she almost hit Sei so I don't think it's going that direction sadly....