@icarushector I'm pretty sure they are all related. Doesn't mean one of them can't feel something ... inappropriate towards a sibling. This ain't the kind of comic where it will likely go anywhere, but it is still an interesting story point.
In chapter 5, MC explained to FL that the teacher is his eldest sister's childhoodfriend (rather close, too). I'm not sure what could make you think he lived in that house, nor that he was their brother...
@icarushector At first I was confused with their relationship too. I thought the homeroom teacher was the eldest sibling and not living with them. So I had to back read, and on chapter 5, Jooho explained to Nari that their Homeroom teacher is childhood friends with his Noona. Meaning they're not siblings or blood related.