What did the mom say!? Are they brothers or not!? Huh?!

that's what i also wanna know

I believe they are blood related, Which explains why Do-Hwa was devastated when his mother answered him. I guess he wanted some kind of excuse to how he felt towards Dojun.

Then again, from his creepy and dirty ass antics, I think he will still go after Dojun, regardless if he was adopted or blood related.