I always assumed the Big brother was also in high school, maybe 1-2 years older at most. But if our main character is a grown man who goes on business trips this whole series suddenly changes for me.
In my head cannon I could explain their twisted romance as youths who don’t know better but if our MC is a grown ass man who’s dating his 13 year old sister? Yikes. And once you’ve seen the R18 stuff double yikes.