B is the father of Q. B has only two children. Q is the brother of R. R is the daughter of P. A is the granddaughter of P. S is the father of A. How is S related to Q?

Given that Q is the brother of R . B is the father of Q : this implies B is the father of both kids Q & R.
It is also given R is the daughter of P : this implies R is female and P is mother because already Father is deduced to be B .

So, now deduced that {B(father), P(mother)} and their kids are {Q(male), R(female)}.
It is also given "A" is grand daughter of P , and 'S' is father of A : this implies {S(male), R(female)} got married and their offspring is {A(female)}. 

Now, the relation b/w Q and S are brother in laws.