Even when you break my toys
Even when you're making too much noise
You will always be my
I am not implying Luna and Discord are related.
However, you don't have to be related by blood to consider someone your little brother.
This is part of the head canon I have. Celestia never trusted Discord, but Luna loved him (as a brother, mind you).
Discord's younger version is pretty much based on
's adorable little Discord design. Actually, she was one of the reasons I finally decided to draw this.