(I). A, B, C, D, E and F are six members of a family.
(II). One couple has parents and their children in the family.
(III). A is the son of C and E is the daughter of A.
(IV). D is the daughter of F who is the mother of E.
What relationship do D and E bear to each other ?
E is the daughter of A and F is the mother of E. So, A is the father of E and hence the husband of F. Now, D is the daughter of F. So, D and E are the daughters of A and F.
Also, A is the son of C. Now, only B remains. Thus, B and C are the parents of A.
Clearly, D and E are sisters.