is the "ectobiological siblings" thing an aspect of all sburb sessions where the players' species has some biological concept of siblinghood?

There’s a difference between Perceived Ecto-relatedness and Actual Ecto-relatedness. Even without the concept of family in any sense, some of the Trolls are still genetically related, they simply don’t care because Troll Biology is weird and pretty much already sort of accounts for that. They’re no different from the Humans, except once the Humans do get told they’re related in pairs things get a bit weird. (See: Freudian Slips, and Sprite^2 Shenanigans). That being said, while genetically related, whether two Ectobiologically related characters begin to act more like siblings or keep just being good friends, is up to them. They weren’t raised together in a family either way, after all.