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Vasqueztion
I think I'll use this saying from CinemaSins, "The power of their boners" changed them. From that world's viewpoint, Ed's and Iris's earlier engagement could be seen more as a political engagement rather than a engagement of love. Although Iris was in love with Ed before, Ed didn't share the same feelings, he probably got engaged out of obligation and had been told the benefits of having a duke's daughter marry him. In comes Yuri who has everything Ed is looking for in terms of personality and temperament(even if he doesn't realize it's all a lie) and falls in love with her over the course of his 'route' in the game, while she is also getting 'loved' by the other three as well. If you look at each of them individually, they all have personalities that SHOULD be noble-like, but their 'love' for Yuri made them act like idiots.
Ed is supposed to be a prince befitting his status, and being in love he wants to make Yuri happy by showering her with gifts and trying to change the kingdom to how she wants it by her 'innocent' suggestions.
The knight son is supposed to be a hardworking knight in training who being praised for his efforts by Yuri, holds down Iris despite her not being violent in the least, something frowned on in knight circles as knights are not supposed to harm unarmed civilians and uphold a code of honor, just because of his 'love' for Yuri.
Bern her own brother denounced her publicly as family, which has huge repercussions to a noble's reputation when spread saying that even family disapproves of her actions, Iris is summarily banished from noble society, with all the things she didn't do blamed on her, all because of his 'love' for Yuri.
The last one didn't do much but he stood by Yuri like the others. But his position had also a heavy influence.
The four combined are a Prince, a Duke's son and her brother, a future kingdom knight, and someone from the church. That covers practically every point that shows they are on the side of 'justice' when they denounced Iris at the beginning of this story. One thing I will heavily enjoy is watching EACH of these idiots realize their stupidity.