Kyoya didn't know when it had happened, but he did know that he had fallen in love with his adopted daughter, Haruhi.
Naturally, he knew not to tell her; they would both be hurt by such a relationship, and it would ruin the Ootori name. (Not like Kyoya and Tamaki's relationship hadn't already tarnished it.)
Kyoya was forced to acknowledge his adopted daughter's feelings for him when he constantly caught Haruhi staring openly at Tamaki and himself.
Haruhi began to inquire about how Kyoya had met Tamaki, and how they had realized that they were in love.
Kyoya found it interesting to hear Tamaki's answers to such questions; it made him wonder if Tamaki needed a psychologist, or if his memory had already started to fade from old age.
Haruhi had quickly gotten bored of her adoptive father's extravagant tales, and started asking Kyoya about his version of events.
It went something like this: "He meant a lot to me, and it was the same for him that it was for me." Kyoya had said it like it was, but he was a little disturbed by the lack of emotion in his voice when he had talked about it.
Haruhi started to talk to Renge more, and Kyoya began to notice that after a while, Renge began to spend most days over at The Ootori-Suoh Estate.
Judging by Renge's squeals and questions, Kyoya determined that the meddlesome girl was trying to get Haruhi and Kyoya to go out.
Kyoya agreed with Haruhi when he overheard her say, "It's hopeless."