The fiance seems to be dominated by something.
Main girl has free will but it changes nothing, the world goes against her; the fiance seems forced to go against her, he has no choice in what's happening or the affair itself. Meaning someone's forcing him on the sister despite him being loyal to the MC.
Could be the case for everyone else too, or if someone among them is doing this then nearly everyone. Would explain why people talk about her in such odd ways. Think there was some reference to someone, her mother or nanny maybe, calling her spoiled even though MC says she wasn't and the person should know that.
Could be the sister doing this but it feels too obvious, kinda run out of suspects after that.