In terms of weight, F < B, D < B, B < E, E < C. So, we have : D < F < B < E < C or F < D < B < E < C. C is not the heaviest. So, A is the heaviest. Thus, the sequence becomes : F < D < B < E < C < A or D < F < B < E < C < A.
D is the brother of F.
B is the mother of D and F.
A, being the heaviest, is the grandfather.
Now, C is a lady and so one couple is AC.
B is a female and so on cannot pair up with C. So, the other couples is BE.

E is the husband of B and B is the mother of F. So, E is the father of F.