I don't think he knows the queen is working with ghosts and stuff. Because he was also quite young when the queen did cruses to Lee nok's mom but regardless, he's either very dumb or very confident like my guy, this is your sister on the line
But they have already found the cursed objects around the palace in the previous chapters and it made a big fuss. Even if he doesn't know that the prince is cursed (but still, there were rumors in the whole country, if you reread the first few chapters. Also he knew that the prince couldn't sleep at night) he should know by now that the queen is working with curses.
Yeah, that'd why I said I don't think he knows the queen is working with ghosts and stuff. Curses he probably could believe in but actual ghosts, I don't think he can believe it because Lee nok didn't disclose that.
I know everyone is obsessed with the love haha but talking about the story, I'm not sure if Kyungjoon's idea would be good (I know what will happen, but still I think about his plan) . He knows that 3 wives have died already and still would push his sister to marry Lee Nok. Dude your family might be quite influental, but you saw already that the Queen is working with ghosts and curses, yet you choose to throw your sister into it? Nah
Help Lee Nok as his royal bestie, not as the brother of his wife... That sound like an either suspicious or very stupid idea.