Prolog: a person is a sibling of himself?

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The actual reason for your problem is (\==)/2 within different/2. It succeeds simply too often. Replace it with dif/2 and you will get the expected behavior. dif/2 is available in many Prolog systems like SICStus, YAP, B, SWI. You can also define a safe approximation in ISO-Prolog like so:

iso_dif(X, Y) :-
   X \== Y,
   ( X \= Y -> true
   ; throw(error(instantiation_error,iso_dif/2))
   ).

Now, should the arguments be not sufficiently instantiated, you will get an Instantiation Error. Prolog aborts the computation and says: I have no idea! Which is far better than pretending it does have an idea while it has none.

Using iso_dif/2 you will still have to place it at the end of the rule. But this time, Prolog will keep an eye on its correct usage.

| ?- iso_dif(a,b).

yes
| ?- iso_dif([a,_],[b,_]).

(8 ms) yes
| ?- iso_dif([a,X],[X,b]).

yes
| ?- iso_dif([a,X],[a,X]).

no
| ?- iso_dif([a,X],[X,X]).
uncaught exception: error(instantiation_error,iso_dif/2)

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