To find So R we want to find the set of pairs (a, c) such that for some person b, a is a parent of b, and b
is a sibling of c. Since brothers and sisters have the same parents, this means that a is also the parent of c.
Thus S o R is contained in the relation R. More specifically, (a, c) E S o R if and only if a is the parent of c,
and c has a sibling (who is necessarily also a child of a). To find Ro S we want to find the set of pairs (a, c)
such that for some person b, a is a sibling of b, and b is a parent of c. This is the same as the condition that
a is the aunt or uncle of c .
