I do understand why people are saying this is incest, but to me it isn't, because they did...

I do understand why people are saying this is incest, but to me it isn't, because they didn't spend their whole childhood together and they were not actually sister and brother as he was not even adopted, he was just with them for a bit more then 2 years, to me it's more like a relationship of a kid looking up to the girl that lives in the house next to his, and he calls her sis more like "noona", that is very common in korean, japanese and chinese cultures...