Re siblings who don't have the same siblings -I have No idea, my kids are still very confused that their cousins have other cousins who are not my kids cousins.
And when they were younger they couldn't understand that their father was still the father of their adult siblings who didn't live at home any longer. For some reason it was fine for me to still be their mother.
But,on the other hand, SS don't seem to understand that our children can have bio siblings on both sides of their birth family and not just their bio mothers side. And allegedly they are adults 