If it was already deduced that Horon and Horan were people who lacked self-confidence and lived on the false praise of others, it was not difficult for Kain to deduce that this was due to the influence of their previous living environment, something that had been deeply engraved in the minds of these two even when they were adults.
This is the effect of childhood trauma.
As for the fact that both of them were descendants of a noble family, this was inferred by Kain based on two things.
First, it is about the magical talent of these two twin brothers.
It's not that rare for Magus to become Adventurers, but most of them are so untalented that they only make a few short steps before returning to the path of ordinary Adventurers. Being able to show off their talents among them, or even become a "Magus", proves that their talent isn't too bad.
