Yeah, I agree with the social norm thing. I mean, even in the 21st century I'd say that it's generally thought of as quite weird to say one doesn't love one's mother as much as or more than any other family member. (I know, I know, present company is immune to that sort of social pressure, but it is still a thing.)

it won't surprise you that I've written fanfiction about Corleone sibling relationships.

Ooooooh! *goes to look*