P is the sister of Q. Q is the wife of R. S and R are brothers. What is P to R?

Let's decipher the relationships:
1. $Q$ is the wife of $R$ ($⟹Q$ is female and $R$ is male).
2. $P$ is the sister of $Q$.

Since $P$ is the sister of $R$ 's wife ($Q$), $P$ is the sister-in-law of $R$.