Oh yes that YouTube interviews, I watched it too... Their reasons is that, they're paying the hosts/prostitutes for the hours and nights so it isn't considered as cheating... it's more of like business... I know, that part of culture specifically the cheating culture of Japan, non of us outsiders of that country could ever understand... and even some Japanese don't too. What's more the irony of it is that, guys are okay to cheat but woman cheating is not good. A youtuber (his name was Matcha something) asked his friends about them seeing other girls, they said fine, it's cool... but when asked if their girlfriends ever tried to see someone out, they answered it's not good because their girl is cheating on them. Watching that even that Youtuber Matcha got confused and was flabbergasted by what his friends said, he just laughed it off saying along the lines of: "What the heck is that? I don't get it"
I honestly think people are being a tad bit too harsh to the story. i thought it was a great read with good characters. people are really hung up on the sister part, but are forgetting which country and culture the story is set in. we don't know the sisters full situation, cus this isn't her story. but it's likely that sis has a contractual marriage. not in the actual contract part but in the we are 2 people that wants to settle down, start a family and grow old together. the ml gave her love, but she knew he couldn't commit. like she said, he didn't chase her. so she married the other man. (to the cheating part, there's literally videos on youtube with japanese people saying "having sex with a prostitute isn't cheating")
anyway love the story ヾ(❀╹◡╹)ﾉ~