P is the brother of Q. R is the sister of Q. S is the sister of R. How is Q related to S?

The correct choice is: “brother or sister”

P, Q, R and S are all children of the same parents: P is Q's brother, R is Q's sister, and S is R's sister, so S is also Q's sibling.

The statements tell us the gender of P (male), R (female) and S (female), but nothing about Q's gender. Q may be a boy or a girl, so Q is S's brother or sister. “brother” and “sister” each assume a gender that is not given, and “son” is not a sibling relation at all.