No, the twin brother(jeong woo not sure of the spelling) nvr know he had a child bcoz his already in the verge of dying..
and to answer your question about how did yeon woo know it was his niece its bcoz dragon species is already extinct and the only dragon species alive are the summer queen and jeong woo (which is now assumed dead) and then jeong woo past down the dragon attribute to his brother which is yeon woo (the mc) but no one knows that in the early chapters so most of them think that the summer queen is the only one left. Yeon woo just accidentally found sesha when his searching for brahm and he knows it was jeong woo's child bcoz sesha has a dragon skin and tail.. besides she inherited yeon woo's naming sense and stubbornness (=・ω・=)
Oh by the way if i remember it correctly atlanta is also a dragon species(im not sure sorry i cant remember)
ok so um this is probably a really dumb question but is he getting revenge because the top rankers killed his brother or is he doing it because his brother is alive and hes climbing the top ranks to find him