A Mirror?
To start with, Yuuhi and Chidori's relationship is somewhat similar to Yuuhi's and Aya's. Yuuhi protects both girls fiercely, argues, teases, and cares for each in a very similar fashion. The only difference being that Yuuhi is fairly more relaxed around Chidori, able to open up and talk about his own problems, whereas it's usually the other way around with Aya. Yuuhi is always there trying to help her and support her, taking up her pain and bearing it himself.
Chidori acts to Yuuhi as Yuuhi does to Aya, selflessly helping and supporting him and his love, doing so perhaps even more purely than Yuuhi [I guess he is still a hormone enraged male afterall].
This isn't totally dependant on the fact that she wants her beloved to be happy, it's also to do with the small girls undying loyalty towards Aya. She believes for a time that Tooya is unsuited for Aya as a partner: he was working for the enemy, had no qualms with killing, and the uncertainty of his past and who he was did little to aid that. Chidori feared for Aya being hurt and thus pushed Yuuhi, who presented a solidity that the girl needed, to be with her. In the short time preceding Chidori's death where Aya is living happily with Tooya, she too pushes Chidori to admit her feelings for Yuuhi.
What I believe this represents is a mirroring of their relationships and feelings. As you might have noticed from the ending of Ayashi no Ceres, Tooya will not live for much longer and 'asks' Yuuhi to take care of Aya afterwards. Because Tooya realises that Yuuhi truly does love Aya, and Aya loves him back...just not as her number one. If that's the case then Yuuhi could very well feel the same about Chidori. However her untimely death cut short the blossom of romance.
Continue: Love?