A, B and C are sisters. D is the brother of E and E is the daughter of B. How is A related to D?

Aunt

E is the daughter of B, so B is E's mother.

D is the brother of E, so D and E share the same mother, which is B.

Since A, B, and C are sisters, A and B are siblings.

Thus, A is the sister of D's mother (B), which makes A the "aunt" of D.