I'm deciding on having Drasel=E9q words for the
groups formed by a parent and a child (in the
different combinations of sex). This popped up
while I thought that we say=20

   "A is the brother of B"
   "B is the brother of A"
=3D=3D> "A and B are brothers"

but we can't say

   "A is the father of B"
   "B is the son of A"
=3D=3D> "A and B are *X"

Of course, the relationships are different,
symmetrical vs. asymmetrical, but that is logic,
not grammar. Note that I could have said
"brother", "sister" and "siblings" in the three
lines of the first example, and it would have
worked too.

I'd like to know if this sounds likely and natural
(if some natlang has it, preferably).

--Pablo Flores