why does this not work

I have this : 

male(roelof).
male(mans). 
male(ronald).
male(jan).

female(chantal).
female(marie).
female(gerda).
female(dagmar).
female(denise).
female(kimberly).


parent(mans,gerda).
parent(mans,roelof).
parent(marie,gerda).
parent(marie,roelof).
parent(dagmar,denise).
parent(dagmar,kimberly).
parent(ronald,denise).
parent(ronald,kimberly).
parent(chantal,tamara).
parent(roelof,tamara).
parent(jan,chantal).
parent(jan,dagmar).

% Looks for the father of a child. Gives the name if found. 
if no father is  found then it gives false
father_child(Child) :-
 parent(Father, Child),
 male(Father).

% Looks for the mother of a child. Gives the name if found. 
if no mother is  found then it gives false
mother_child(Child) :-
 parent(Mother, Child),
 female(Mother).

% Looks if two person has the same father. 
% Gives true if a person is a father of both persons 
% Gives false if no person is a father of both persons.
same_father(Child, Sibling) :-
  parent(Father,Child),
  parent(Father,Sibling),
  male(Father).

% Looks if two person has the same mother. 
% Gives true if a person is a mother of both persons 
% Gives false if no person is a mother of both persons.
same_mother(Child, Sibling) :-
  parent(Mother,Child),
  parent(Mother,Sibling),
  female(Mother).

% Looks if there are siblings of a person. 
% Persons are siblings if they have the same father or
% if they have the same mother and not the same father.
siblings(X,Y) :-
        (   same_father(X, Y),
        X \= Y
    ;   same_mother(X, Y),
        \+ same_father(X, Y)
    ).

% Displays the output of siblings(X,Y) and takes care that 
% there are no duplicates.
display_siblings(Person) :-
      findall(Person-Y, (siblings(Person,Y),Person @< Y), Sibs),
       display_the_siblings(Sibs).

% Display a message if there are no siblings found. 
display_the_siblings([]) :-
       write('Er zijn geen zussen/broers bekend').

% Displays a message if one sibling is found.
display_the_siblings([X-Y]) :-
        write('The enigste broer of zuster is '),
        write(Y).

% Display a message if there are more then 1 siblings found.
display_the_siblings([H|T]) :-
        XYs0=[H,T],
        write('Alle zusterparen zijn : \n'),
        forall(( member(X - Y,XYs0)
       ),( format('~w en ~w. \n',[X,Y])
        )).
     

But when I do display_siblings(kimberly) I see a message Er zijn geen zussen/broers bekend
That's wrong Denise is a sibling of Kimberly.

Where does I make a error.

Roelof

 		 	   		  
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