Explanation:

- P is the father of Q, meaning P is the parent.

- R is the son of S, making S the parent of R.

- Q is the sister of R, implying Q and R are siblings.

- T is the brother of P, so T and P are siblings.

- We need to determine how S is related to T.

Since R, the son of S, is also Q's sibling, S must be the other parent of Q. Therefore, S is P’s spouse. As T is P’s brother, that makes S T's sister-in-law.

- Option 1, Brother-in-law, is incorrect.

- Option 2, Daughter, is incorrect.

- Option 3, Sister-in-law, is correct.

- Option 4, Brother, is incorrect.