I agree, I think Yuri is assuming that the teacher and BIL are still in a relationship or were in a relationship while BIL and wifey were still together. Basically, cheating. But, again, that's Yuri's problem, he's assuming and jumping to a lot of conclusions without even listening.
My theory is Yuri is secretly hoping that they are in fact cheating so he'd have a chance at his sister (what a weird sentence, oh brother), that's why he's so insistent and actively antagonizing the teacher.
Same I also think that yuri is not at fault and not to fully hated person he is being reasonable here he care about his sis people need to put themselves in the place of yuri just to feel how would Hey react if they find out their BIL had relationship with a man it would be same reaction as yuri or may be even worse
Am I the only one who doesn't think that Yuri is that much of an asshole and not totally unreasonable?
I don't if it's because I am a terrible person or what but I think I would have acted the same way Yuri did if I was in the same situation . I would have been angry at my crush's spouse and be even more angry if I thought they were cheating a person who I know and respect , but instead of confronting the spouse in fear of ruining their marriage if I got it wrong I would have confronted the other person to confirm things.
But there are things that I also am angry at Yuri for like bruh you BIL can also be bi?! And if the teacher likes BIL it doesn't automatically means that the guy is cheating on the wife they can also be exes and he is married so what does it matter who he had previous relationships with?! And have some sympathy don't be so violent the teacher looks like he hadn't eaten and slept for days! Ugh!
Sorry for the long rant∑(￣□￣;)