Question

A is the only brother of b p is the only nephew of b how is p related to a

Answer 1

if p is female the answer is niece

Answer 2

As A is only brother of B, he is male.

P is the nephew of B, thus P is male too.

So then, P will also be the nephew of A.

Or it can be said that A is the uncle of P.

FYI : B could be both male and female, in that case B could be uncle or aunt of P.