Out of all the mangas to troll, this was not it. Also it's not funny. Whoever did this, don't quit your day job to be a comedian. I am afraid you will starve to death.
If it bothers you, you should cut contact with her and block her on social media. I do find it weird that your friend didn't tell you that her sister is dating your ex. But the sister really doesn't have an obligation towards you. The sister is not your friend. So if the whole situation makes you uncomfortable then cut your friendships and move on,......
Yuri really has a low opinion of Mr Lee. I hated that he was slut shaming Mr Lee in his head especially when he did those things during a dark period of his life. This is one of those few age gap relationships where the younger partner is more toxic than the older one. Usually it's the older one who is more possessive and abusive.