% give some facts
male(bob).
male(charles).
male(joe).
female(alice).
female(claire).

father(bob, charles).
father(bob, claire).
mother(alice, charles).
mother(alice, claire).

sibling(joe, bob).
sibling(bob, joe).

% rules
parent(X, Y) :- mother(X, Y).
parent(X, Y) :- father(X, Y).
sibling(X, Y) :- parent(P, X), parent(P, Y), X \= Y.

brother(X, Y) :- sibling(X, Y), male(Y).
sister(X, Y) :- sibling(X, Y), female(Y).

son(X, Y) :- parent(X, Y), male(Y).
daughter(X, Y) :- parent(X, Y), female(Y).

uncle(X, Y) :- parent(Z, Y), brother(X, Z).