I like the two paths they take to deal with what happened to Discord. Celestia remembers him for who he was, almost as if in denial that the being of chaos that they faced was of the same person, and Luna tries to desensitize him so as to erase the pain of losing him.
And now you lost me. Great art in such a short time from the season premiere, but Discord as a brother to Celestia and Luna is just too farfetched for me even in terms of fanon. I can deal with the Celestia shipping with Discord, but relation is the line for me I guess.
I mean no offense, you can do what you want. It is just too off for me. Discord in my view was the ruler of his own world before Celestia and Luna became the symbolic force to stopping him bringing forth an age of ponies. Though others will view things as they see fit which I am fine with.
It has been shown that any of the different pony species can procreate with each other the baby Cakes are a good example. Or maybe Celestia and Luna were once unicorns or pegasi and had to earn their alicorn status like Twilight did. Who knows.