There's this manwha or manhua (not sure) I have read before, mc has to go to cram school or boarding. in the early chapters there was this scene where mc hides on the side of the bed from his sister that is having sex with a dude, its light off. Then he gets a boner and the sister thinks its her dildo and starts playing with it while doing the dude. Theres a lot of funny moments but i forgot the title.

I'm guessing it's hentai? But I also don't know about it. Sorry.