parent(X,Y) :- mother(X,Y).
parent(X,Y) :- father(X,Y).
sibling(X,Y) :- mother(M,X),mother(M,Y),father(D,X),father(D,Y),X \== Y.

haschildren(X) :- parent(X,_).

grandparent(X,Y) :- parent(X,M),parent(M,Y).

cousin(X,Y) :- parent(A,X),parent(B,Y),sibling(A,B).