I go into more detail on The Mongola Family relationship in Josef Finds A Fluffy if you want to give that a read but essentially, Fitzgerald Mongola is a vile, absusive, racist prick who tried to bully his sons into following his white supremacist bullshit. Josef didn’t take to it - he believes he’s superior to everyone - and Jonathan did believe in it until he met Martha - a black woman - realised his dad was full of shit, and married her.
Both of the brothers HATE their father and take every opportunity to piss him off.