I remember coming across a review on novelupdates about a modern showbiz (or maybe school life?) novel saying that the story was a bit similar to the manhwa "The Beast Must Die." Quick summary from whatI remember for The Beast Must Die: Mc's sister committed suicide and he wants to join this secret club in order to get revenge. In order to be accepted, he needs to pretend to ml's partner and they need to go all the way in front of all the club members. If anyone knows anything that sounds similar or has seen the review before, plz share!
I read a novel that is very similar to your description. It's called "Life after Marrying my love rival" https://www.novelupdates.com/series/life-after-marrying-my-love-rival/