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).                      male(william).
parent(thomas,william).                    male(thomas).
parent(thomas,sally).                      male(jeffrey).
parent(thomas,jeffrey).                    male(andrew).
parent(sally,andrew).                      male(steve).
parent(sally,melanie).                     male(patrick).
parent(andrew,joanne).                     male(john).
parent(jill,joanne).                       male(michael).
parent(joanne,steve).                      male(frank).
parent(william,vanessa).                   male(george).
parent(william,patricia).                  male(morty).
parent(vanessa,susan).                     male(jerry).
parent(patrick,susan).                     male(kramer).
parent(patricia,john).                     male(aleksandr).
parent(john,michael).                      male(alessandro).
parent(john,michelle).                     male(aleksander).

parent(frank,george).                      female(tina).
parent(estelle,george).                    female(sally).
parent(morty,jerry).                       female(melanie).
parent(helen,jerry).                       female(joanne).
parent(jerry,anna).                        female(jill).
parent(elaine,anna).                       female(vanessa).
parent(elaine,kramer).                     female(patricia).
parent(george,kramer).                     female(susan).
                                           female(michelle).
parent(margaret,nevia).                    female(estelle).
parent(margaret,alessandro).               female(helen).
parent(ana,aleksander).                    female(elaine).
parent(aleksandr,aleksander).              female(anna).
parent(nevia,luana).                       female(margaret).
parent(aleksander,luana).                  female(ana).
parent(nevia,daniela).                     female(nevia).
parent(aleksander,daniela).                female(luana).
                                           female(daniela).

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

nex(sister(vanessa, andrew))
nex(sister(elaine, george))
nex(sister(sally, thomas))

nex(sister(andrew, melanie))
nex(sister(patrick, vanessa))
nex(sister(michael, michelle))

nex(sister(vanessa, vanessa))
nex(sister(sally, sally))
nex(sister(daniela, daniela))