male(albert).
female(alice).
male(edward).
female(victoria).
male(john).
male(joseph).
female(jane).
female(ann).
parent(albert,edward).
parent(victoria,edward).
parent(albert,alice).
parent(victoria,alice).
parent(john,albert).

% Lecture slide 13
sibling(X,Y) :- parent(P,X), parent(P,Y), \+(X=Y).