I checked out the manual and the FAQ, but still don't get the relationship. The child concept makes sense, but what is a sibling? Some kind of second-cousin-twice-removed weirdness? Or do people use the terms child and sibling interchangeably?
Siblings are 2 or more nodes with an equal spot in the family tree. None is superior nor subordinate so they do not 'inherit' permissions from each other, inherited permissions come from the parent node.
Siblings can have child nodes
It all comes down to the incredible permission stacking system that was a little tough for me to understand in the beginning.