john is a child of sue
john is a child of sam
jane is a child of sue
jane is a child of sam
sue is a  child of george
sue is a child of gina

john is male
jane is female
june is female
sam is male
sue is female
george is male

if X is a child of Y then Y is a parent of X
if X is a child of Y and Y is male then Y is a father of X
if X is male and Y is female then X is the opposite sex from Y
if X is male and Y is female then Y is the opposite sex from X
if X is a father of Y and Y is a parent of Z then X is a grandfather of Z