are they actually uncle and nephew? what’s up w that lol

NO, not actually.
Jinwon's sister was into older guys and ended up marrying his dad which was her boss at the time. So there's NO blood relation there. Plus (I can't remember how to spell his name, but the son 'Gaughyum???'(⌒▽⌒)) was already an adult also at the age of 20, and Jinwon was around almost 30, (I think 27 or 29?).