I feel like he's an Ionian, not a Demacian. Could be related to Irelia, Yi, Lee, or Karma, in order of suspicion on my part. Irelia's lore mentioned she had a brother if I remember correctly, Yi had his entire village killed off, but it mentioned nothing about his family being killed, Lee has a sister for sure, so it's not unlikely he would have a brother, and Karma has had virtually no exploration of her past other than Singed went huehuehuehue all the way through her village.
Also, the language is much more flowery than how most Demacian lores have been written. Demacia champions have always had a very military vibe to them; they're often brisk, to the point, and with little bandying of words. Demacians have always had the actions of betrayal befits punishment, and the lore so far indicates an attempt at Yasuo's redemption, not retribution.
Xin, while it seems like a possible choice, has never been a poet of any sort, and he doesn't have the lore necessary to support such a character in my opinion. He was rescued alone from the Noxian arena by the king of Demacia. He has never indicated he had family ties, and his speech patterns in the game are fairly brusque and to the point, much unlike the passage in the reveal.
Plus, the current art SCREAMS of Ionian heritage. Seriously, you could probably facebomb Yasuo's face on to Ionian Yi and it would look hella similar to that picture.