Predikat sister(X, Y) velja natanko takrat, kadar je X sestra od Y. Primer klica:

?- sister(X, patricia).
  X = vanessa.

Povezave so podane s predikatom parent/2. Predikata male/1 in female/1 določata spol vsake osebe (vozlišča).

parent(tina,william).
parent(thomas,william).
parent(thomas,sally).
parent(thomas,jeffrey).
parent(sally,andrew).
parent(sally,melanie).
parent(andrew,joanne).
parent(jill,joanne).
parent(joanne,steve).
parent(william,vanessa).
parent(william,patricia).
parent(vanessa,susan).
parent(patrick,susan).
parent(patricia,john).
parent(john,michael).
parent(john,michelle).

parent(frank,george).
parent(estelle,george).
parent(morty,jerry).
parent(helen,jerry).
parent(jerry,anna).
parent(elaine,anna).
parent(elaine,kramer).
parent(george,kramer).

parent(margaret,nevia).
parent(margaret,alessandro).
parent(ana,aleksander).
parent(aleksandr,aleksander).
parent(nevia,luana).
parent(aleksander,luana).
parent(nevia,daniela).
parent(aleksander,daniela).

male(william).
male(thomas).
male(jeffrey).
male(andrew).
male(steve).
male(patrick).
male(john).
male(michael).
male(frank).
male(george).
male(morty).
male(jerry).
male(kramer).
male(aleksandr).
male(alessandro).
male(aleksander).

female(tina).
female(sally).
female(melanie).
female(joanne).
female(jill).
female(vanessa).
female(patricia).
female(susan).
female(michelle).
female(estelle).
female(helen).
female(elaine).
female(anna).
female(margaret).
female(ana).
female(nevia).
female(luana).
female(daniela).

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