father(tom,dick).
mother(tom,judy).
father(dick,harry).
mother(dick,mary).
father(jane,harry).	
mother(jane,mary).
parent(X,P) :- mother(X,P).
parent(X,P) :- father(X,P).
granny(X,G) :- parent(X,Y), mother(Y,G).
sibling(X,Y) :- mother(X,M), mother(Y,M),
		father(X,F), father(Y,F).