A is father of C and D is son of B. E is brother of A. If C is sister of D,…

Kinship rule: the wife of a person's brother is called that person's sister-in-law; to name the relation between two people in a family puzzle, first fix each person's gender and parental/marital links, then apply the matching kinship term.

Applying this to the puzzle:

1. A is the father of C, and D is the son of B — so A and B are the two parents in this family.

2. C is the sister of D, which means C and D are siblings, both children of A and B — so A and B are husband and wife, the parents of daughter C and son D.

3. E is the brother of A.

4. Since B is A's wife and E is A's brother, B is the wife of E's brother.

Cross-check: by the kinship rule above, the wife of one's brother is one's sister-in-law, so B is confirmed to be the sister-in-law of E — the child-generation role (daughter) and the male-only roles (brother, husband) do not fit B's position in the family tree.