nope they explained it in the notes, heejoo's mother and Yuri's father are siblings making heejoo and Yuri cousin. yup this man is in love with his cousin LMAO, but I kinda get why, she's the only one who has shown him kindness in his abusive household, so he held affection for her but I guess it turn to romantic affection yikes
well jiri thought that because he didn't know that heejoo was adopted into Yuri's family, and just recently jiri and the readers (us) just found out about the family situation so you're fine, I also thought she was Yuri's sister too. although it doesnt make it better thats its his cousin instead of sister this time lol
Wait yuri is in love with his sister wtf