All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Wife`s sister is temporarily forbidden in marriage to her sister`s husband. i.e. she is a temporary Mahram to him; whether he has consummated the marriage with her sister or not, and this differs from being permanently forbidden in marriage to him (Permanent Mahram). What is forbidden, here, is to be married to two sisters at the same time.
Muslim jurists have stated that being a temporary Mahram to her sister`s husband doesn`t mean that she is forbidden in marriage to him in a permanent sense. Therefore, it isn`t permissible for her to be alone with him, shake hands or not wear Hijab before him. Rather, she should be keener on covering herself before him than before anyone else. This is because the Prophet, peace and blessings be upon him, said, "Beware of entering upon the ladies." A man from the Ansar said, "Allah's Apostle! What about Al-Hamu the in-laws of the wife (the brothers of her husband or his nephews etc.)?" The Prophet(PBUH) replied: The in-laws of the wife are the death itself."{Related by Bukhari and Muslim}. Commenting on this narration, An-Nawawi said, "A woman should beware of her in-laws more than anyone else for evil and temptation are more likely to come from them as they can enter her house and stay in seclusion with her without raising suspicions, unlike a stranger or a non-relative. Al-Hamu refers to the male relatives of a woman`s husband."{Shareh Muslim, 14/154}. Ibn Daqeeq said, "A Mahram with whom a woman is allowed to travel and stay in seclusion is every man who is permanently unmarriageable to her."{Ihkam Al-Ahkam, 2/57}.
In conclusion,although the wife`s sister is temporarily forbidden in marriage to her sister`s husband-whether he has consummated the marriage with her sister or not-she isn`t a Mahram to him. And Allah knows best.