$P$ 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 step-by-step:
1. $Q$ is the wife of $R$ ($⟹Q$ is female, $R$ is male).
2. $P$ is the sister of $Q$ ($⟹P$ is female).
3. Since $P$ is the sister of $R$ 's wife, $P$ is the sister-in-law of $R$.