% file: genealgy
% author:

%      Parent		Child
parent(george,		elizabeth).
parent(george, 		margaret).
parent(elizabeth,	charles).
parent(elizabeth,	anne).
parent(elizabeth,	andrew).

female(elizabeth).
female(margaret).
female(anne).
male(george).
male(charles).
male(andrew).

sister(X,Y) :-
    parent(Z,X),
    parent(Z,Y),
    female(X),
    X /= Y.

brother(X,Y) :-
    parent(Z,X),
    parent(Z,Y),
    male(X),
    X /= Y.

