What should a wife do if her brother-in-law's wife mocks her, and they live in the same house?

Your brother's wife must fear Allah in what she says and refrain from mockery, backbiting, and slander. If she does not stop, you should ask her husband to discipline and restrain her. Your patience with her harm and repelling her evil with good is better for you, in accordance with His, the Almighty's, saying: "The recompense for an evil deed is an evil one like it. But whoever pardons and makes reconciliation - his reward is due from Allah. Indeed, He does not like wrongdoers," and His saying: "And not equal are the good deed and the bad. Repel [evil] with that which is better; then suddenly, he to whom between you and him was enmity [will become] as though he was a devoted friend. But none is granted it except those who are patient, and none is granted it except one having a great portion [of good]."