I think it's probably that for Aya felt completely different to talk to Mitsuki than it felt to talk to Onii-sama. He was technically an older, cooler, super hot stranger that was into the same stuff as her and totally her type.
Koga-san is just easier to talk to, even if she is cool and all that, Aya also knows she is a nervous wreck and her friend. Of course she has a major crush on Koga-san too, the feeling is just different tho.