He's his nephew because Wei was basically adopted by his grandparents and raised with his mom and Jaing uncle like siblings. (Not a spoiler, this was mentioned like 100 chapters ago or smth. Lol.)
Yes. Yes, I do comment in paragraphs. 
Dads, uncles, brothers, oh my! Check out list of (mostly) BL with kids: https://mangabuddy.com/manga-list/46295
Dads, uncles, brothers, oh my! Check out list of (mostly) BL with kids: https://mangabuddy.com/manga-list/46295