We know it in the chapter 31. It's because he is born as a dragon and not a snake like the parents would. And he is born as a black dragon which is even worst for the parents. His eyes are a darker golden yellow than the emperor, (it means that he is or will be more powerful than his father in the future) and it's for that that he blindfold himself because he thinks his eyes are a nuisance to everybody in the castle. But the little brother is '' perfect '' for the parents since he is a white snake with golden eyes that are the same tint as the emperor.
I tried to explain what I remember 😅