Honestly claude and his bro’s relationship didnt look that bad when they were children :...

I think it was when Jeanette's mother came into the picture.

I feel like it was more when like claude was shown to be better than his bro in terms of magic or idk. So I guessed he felt inferior to his little bro, thus, from then the relationship went down. I think it was stated in the earlier chpt, I think in chpt 23.