Wait

No no
When he said Whose child is it, he meant the dad
Whose child did fl have, something like that

If I recall correctly, back when she first started living with them, he looked into her past and mused that there was no proof she had ever given birth to a child—so I don't know if that was just his speculation or if he actually knows something.

Yes yes you're right I remember he had done a background check and figured that the child wasn't hers. So he knows that much for sure.. I think it might take some more time for him to figure who's child it is... Perhaps if the step sister tries to get even closer, which she will lol.. maybe he'll dig into her past and find something about Minho? It's a possibility :)

Yeah this makes the most sense. Forgot he did a background check so he's probably had an idea for a while

That could possibly be it, but because he says that right after thinking about the stepsister kinda implies a connection. And, granted this is the translator's choice, the use of "whose child" instead of just saying father makes me think otherwise.