Commentary for Kiddushin 134:14

The Talmud raises a difficulty concerning the analogy of a wife’s sister to the case of a brother’s wife. If the brother dies without children, then there is a mitzvah to marry her. So maybe in this, the kiddushin would be valid (even not in a case of yibbum) for she is not like a wife’s sister, who is never permitted. There is never a mitzvah to marry one’s wife’s sister.