That's what I'm confused about, when I read ch 1-2, I'm pretty sure Heejo is Yuri's older cousin, not older sister as in blood related... tho starting from the new translator ch 3, it is referred as sister... so now I'm confused,
Ofc Yuri will still call her Noona, but I need to confirm whether she is cousin or blood sister, I think it's pretty fucked up if he fell for direct blood-related fam

Yuri is in love with his cousin (Heejoo), and Heejoo married BIL who the teacher was in love with. Yuri assumed that the BIL was also gay because of this and was worried that Heejoo is being deceived, that’s why he wanted to let Heejoo knows that BIL is gay, or bi. But he’s assuming things based off of just the teacher’s response. In truth, it was a one-sided love from the teacher, but Yuri at this point just wanted to break off the marriage any way he can because he loves Heejoo. The sister probably knows Yuri loves Heejoo too that’s why she’s being all pissy, bc she doesn’t want him to interfere with Heejoo’s love life anymore.