All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
It is temporarily forbidden for a man to marry his sister-in-law, but this doesn`t make her a Mahram (Permanently unmarriageable) of his. The majority of the Muslim scholars have clarified this through the following rule: "Being temporarily forbidden from marrying someone doesn`t make it unlawful" Therefore, it is impermissible for a woman to expose herself before her sister`s husband as she does before her Mahrams.
It is also impermissible for her to stay in seclusion with him or shake hands; rather, she should be more keen on covering herself before him than before anyone else. This is because the Prophet (PBUH) 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 foreigner or a non-relative. Al-Hamu refers to the male relatives of a woman`s husband." {Shareh Muslim, vol.14/pp.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, vol.2/pp.57}. And Allah, The Almighty Knows Best
