P and Q are brothers. R and S are sisters. The son of P is brother of S. Q is related to R as :

P's son is a brother of S, so S is P's child, and since S is a sister of R, S is P's daughter. R is S's sister, so R is also P's daughter. Q is P's brother, which makes Q the paternal uncle of R; Q cannot be R's father or brother, because those relations belong to P and to P's son.