P and Q is a married couple. X and Y are sisters. X is the sister of P. How is Y related to Q?

Sister In law

Let's analyze the relationships:

• P and Q are a married couple.
• X and Y are sisters.
• X is the sister of P.

Therefore:

• Since X is P's sister and Y is X's sister, Y is also P's sister.
• Q is P's spouse.

Hence, Y is Q's sister-in-law.