The ML's elder sister and her husband -- they were so irritating that I was hoping for a much much more satisfying form of retribution than how it was dealt with in the book.
It really grinds my gears how the ML's mother was so easily influenced by the ML's cousin's words and easily blames everything on the MC at the slightest provocation. If it weren't for her, most of that cousin's schemes wouldn't have come to fruition. At the same time, I also had the nagging suspicion that if they had told her of the cousin's true nature earlier then everything might've been resolved more quickly. Their main reason not to reveal it earlier was that they were worried about her health... and yet everything was revealed in the end anyway... without any real repercussions to her health.