Question

A&B are brothers,C&D are sisters.A son is D's brother then how B is related to C

Answer 1

A and B are brothers
D is the daughter of A.
Thus, C is also the daughter of A.

As A and B are brothers,
  therefore, B is the uncle of C.

Answer 2

As A's son is D's brother therefore
D is a daughter to A
So C should also be a daughter to A
A and B are brothers 
So C's uncle is B(C's father's brother)