When did he find out MC is a cat? Is it the uncle's call or earlier I don't remember

in the earlier episodes, because Yumin's heat cycle medicine fell from his pants, and Taejun also being an hybrid knew immediately what they were for. The grandpa doesn't know that Yumin is an hybrid, he just knows is a man, and they have special medicine to get man pregnant, but since their both hybrids all they have to do is go through a heat cycle together and PUM, baby.

Thank you !