I believe its SeoAn's brother
It's obviously his dad, if the aunt was young and SeoAn was a young teenager, that adult man at the door was his father. SeoAn's brother wouldn't be a bulky tall man at that time, he would've been like 18?
His brother wears eyeglasses. So, it must be his dad.
My apologies. I thought we're talking about the guy with the glasses at the ending as SeoAn's dad
is that SeoAn's dad because wow, hi sir