My wife's sister is married to a Hindu. My wife wants to visit her sister.

Answer

Allah Ta'ala says in the Qur'aan Shareef: "O People of Imaan, do not take as friends your fathers and your brothers if they prefer kufr over Imaan. And, those among you who befriend them, verily they are the transgressors." (Surah Taubah, Aayat 23) Your wife's sister is not married to the Hindu. Marriage with a Hindu is not valid in terms of the Shariah. She is living in a state of adultery with the Hindu. She is no longer a Muslim. She has become a murtaddah (renegade). It is haraam for your wife to visit her. It is haraam for you to allow your wife and children to visit the murtaddah. It is not permissible to partake of her food or to have any kind of social or family relationship with her. It is Waajib to sever all ties with her.