Ok, some stuff got lost in adaptation and translation from the novels, not the text itself but the intent, fake mc's reaction to the confession is more dismissive and almost comical on how they guy gaslights himself: "if I can be liked by Azusa then I just need to comunicate my feelings for shiho better, right? Thanks Azusa, I needed that ...now look at how cool your big bro is while I go confess to shiho, cheer me up!"
That's why Mc gets angry saying that the sister's feelings was medicine that could have turned around everything for fake mc but he just crushed them and spit them out, so it became poison.
At least now Lil sis is free and the final showdown is next.