i personally think it's a response to the environment but developing very early on (in the first 1-3 years of one's life)
most of my friends follow a certain pattern when it comes to the stacking's of their parents (and siblings) and their own.
exceptionally often, they have the same flow, same subtype (e.g. mom: sx/sp (sp subtype) with son: so/sx (sx-subtype) ; mom: so/sx, dad: so/sp?, son 1: sp/so, son 2: sx/sp <- all same subtypes) and have the blindspot of the mother (usually) or their older sibling as their primary instinct.

there are lot of exceptions to this though, especially when the children have a huge age difference or children were not raised closely in early childhood by their parents, or health, environment plays into this & then also very different combinations show up. this is just my personal observations, rather than a general rule.

really all of my closest friends follow this pattern, except one of them. it doesn't apply to myself at all, too (my mother is so/sp with strong sp) and it caused a lot of strain in the relationship with my mother. and in both our cases (my friend and me) we happened to spend a lot of time with various other people in our very early childhood (or were raised by them, basically)

i have also seen twins who have different instinctual variants, btw ... i'm not sure how often this happens, though.