Can anyone give me some hints about this question(Family tree)?

Question

It comes from my homework assignments. There is a family tree

                             a  +  b
                         /   |    |   \ 
                        c+u  d+c  e+w  f
                     / | \        / \
                 m+x  n+y  o      p  q
                  |
                  r

a and b is the oldest. and every married people the second person is not part of the original family. Now I need to write the spouse, sibling, children ,grandchildren, parents and grandparents function.

I wrote the list as below: ( (father mother) chlid1 child2 child3)

(((a b) c d e f) ((c u) m n o) ((d v) nil) ((e w) p q) (f nil) ((m x) r) ((n y) nil) (o nil) (p nil) (q nil)  )

I have some problems with sibling function, here is my code.

(defun sibling  (arglst lst)  
 (cond
        ((eql 
             arglst (cdr (car lst))) 
                 (rest (cdr lst))
         )
   (T (sibling (rest lst) arglst))

)

I knew it was wrong, but I don't how to revise it.. and I also need some help with other functions. hope can get some hints from u guys.