/* mother (mother, child) */
/* father (father, child) */

mother(martha,stephen).
father(charlie,stephen).
mother(marth, greg).
father(charlie,greg).
mother(carolyn,charles).
father(rob, charles).
mother(carolyn,chris).
father(rob,chris).
mother(nancy, justin).
father(bob,justin).

sibling(X,Y) :- mother(M,X),mother(M,Y),father (F,X), father(F,Y).