female(pam).
female(liz). 
female(ann). 
female(pat). 
male(tom). 
male(bob). 
male(jim). 

parent(pam,bob). 
parent(tom,bob). 
parent(tom,liz). 
parent(bob,ann).
parent(bob,pat). 
parent(pat,jim).

mother(X) :- parent(X,Y) , female(X).
father(X) :- parent(X,Y) , male(X).
siblings(Y,Z) :- parent(X,Y) , parent(X,Z) , not(sameperson(Y,Z)).

sameperson(X,X).