% Define the parent relationship
parent(alice, bob).
parent(alice, charlie).
parent(bob, david).
parent(bob, emily).
parent(charlie, frank).
parent(charlie, grace).

% Define the male and female relationships
male(bob).
male(charlie).
male(david).
male(frank).

female(alice).
female(emily).
female(grace).

% Define the sister relationship
sister(X, Y) :-
    female(X),
    parent(Z, X), % X and Y have the same parent
    parent(Z, Y), % Y and X have the same parent
    X \= Y.      % X and Y are different individuals

% Query to find sisters
?- sister(X, Y).