Number of the letters skipped in between adjacent letters of the series starting from behind are increased by one. Which of the following series observes this rule ?
(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 ?