P has a son and a daughter. S is the mother of T. S is R's spouse. Q and R are children of P. Then how is Q related to S?

P has one son and one daughter. Q and R are the two children of P, so one of Q, R is male and the other female. S is R's spouse and S is the mother of T (so S is female). Since S is R's spouse and S is female, R must be male — so R is P's son and Q is P's daughter. Therefore Q is the sister of R, and R is the husband of S. Hence Q is the sister of the husband of S.