Sayyiduna Uqbah bin Amir al-Juhani (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not enter upon women (in privacy), even if they are your brothers' wives." We said: O Messenger of Allah, what is the ruling about the husband's brother? He (peace and blessings be upon him) said: "The husband's brother is death."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 732