1) People confuse cheating and "NTR"
2) Why do they identify with the cheated on brother which isn't even onscreen? It's like they're the ones being cheated on. Are they that insecure?
3) They probably like worse stuff, like loliporn or dubious consent, but somehow "cheating" is a huge trigger for hate, like, unforgivable.
I wish people would appreciate how in just 7 pages, Sumiko Arai summoned a whole story and background and enough material to write 5 volumes. Not mentioning the art.