Each of the capture targets have an alternate reason for doing all that shit to Emi, which is what gives Remi 100% moral high ground for all the horrid shit they'll get when her revenge finally goes off.
The common ground for all of their betrayals is that they were all jealous of Emi's achievements and basically wanted to "knock her down a peg". It also served as a way to remove her from being the prince's fiancé so that they could have her to themselves instead.
In the case of the prince, his intention was to have her rebuild her achievements and then use that as a justification to re-engage her so that in the future when someone says how amazing she is, he wants to use this to say "yeah, you know she fucked up big this one time".
As for the brother, he is 100% affected by the drug, however as many previously have pointed out, the drug only raises your affection to that 1 specific person and NOTHING ELSE. His plan, much like the prince's was to have Remi lose everything, rebuild it all, then use it as a justification that since she made such great achievements, that she could return to being a duchess, being his wife instead. He also plans to use the fact that the prince broke the engage to prevent them from re-engaging and is in a rush to scoop her up the moment he can.
The bottom line is that they were all horrible people that did not deserve ANYTHING Emi gave to them. After revenge, they are all in worse situations then they were before Emi helped them but with the added bonus that they alienated they person they care for most in the entire world and has to live with the fact they will never be part of her life anymore, from the dirtiest pits of the gutters they end up in of course.