because their grandfather is finally about to die.
it hasn't been shown yet but I think his brother was also abused in his grandfather's house. and now that the old man is in his death bed, his brother can now move with less restrictions (less cuz the old man's not dead yet)
He was not allowed by his grandfather to get in touch with his brother (grandfather does not like how Dojun looks like his mom and "the reason" for his son's death, so there's resentment and the likes). So to protect Dojun, he supported him financially through his uncle, while acting like the perfect grandson in front of the grandfather. He's only now meeting him since the grandpa is now weak and about to pass away, plus he has established himself, so he can support Dojun now.
rightt, and once the old man die, he planned to take in dojun.. but,, he didn't expect that dojun is already living with his husband
.
Please remind me again why did his brother only come to meet Dojun after 10 years