Their mother is on Joshua's side but doesn't like one more than the other much. This is Runcandel where the nannies raise the children from the next day after birth and the mother meets them again after they turn 10.
For Rosa, their mother, everything is a test to their children even showing her love or favouritism so the six idiots who gave up on the fight for the Patriarch's seat are seen as losers in the family (the two boys that had to give Jin their swords in the past and are usually shown together -forgot their names-, Myu and Anne -the two that laugh and usually bully the twins-, and the Tona twins -who are more on Jin side the more time passes-). In the case of the Patriarch, he's hard to read/deal with to most siblings except 1°, 3° and 12° flagbearer (Luna, Lundia and Jin) and Jonah -who works for the nameless king(?)- but he holds the same policy(?) as Rosa when it comes to their children... it's basically Runcandel's policy so... yeah... ╮( ̄▽ ̄)╭

I kinda forgot half the plot but before re-reading does Jon's hate him.? Or is she favouring Joshua and that's why she's not liking him.? I read chap 203 and am confused by the interaction between them