Q’s father is B’s son-in-law. C is Q’s sister and the daughter of P. P is the maternal aunt of D. How is P related to B?

C is Q's sister and P's daughter, so P is a parent of Q. Q's father is B's son-in-law, i.e. married to B's daughter. P is therefore Q's mother, which makes P B's daughter. (P being D's maternal aunt only confirms that P is female.)