Question

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

Answer 1

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)

Answer 2

B IS UNCLE OF C, BECAUSE A AND B ARE BROTHER.