predicates
    nondeterm grandparent(symbol, symbol).
    nondeterm parent(symbol, symbol).
    nondeterm kid(symbol, symbol).
    nondeterm sibling(symbol, symbol).
clauses
    /*Dad's branch*/
    parent("Sergey","Evgenij").
    parent("Tatyana","Evgenij").
    parent("Evgenij","Kirill").
    parent("Evgenij","Bonya").
    /*Mom's branch*/
    parent("Vladimir","Oksana").
    parent("Nina","Oksana").
    parent("Oksana","Kirill").
    parent("Oksana","Bonya").
    /**/
    sibling(F,S):-parent(M,F) and parent(D,F) and parent(M,S) and parent(D,F).
    kid(C,P):-parent(P,C).
    grandparent(GP,C):-parent(GP,P) and parent(P,C).
goal
    grandparent("Vladimir", "Kirill").