All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The husband of your husband’s sister is a non-Mahram to you. It is impermissible to be in seclusion with him or to remove your Hijab before him, even if he were a very righteous person. What would be the ruling then in the case of him being a drinker! There should be no obedience to the husband in terms of disobeying Allaah. We advise your husband to fear Allaah, The Almighty, and to help you obey Allaah. One of the aspects of obeying Allaah is to avoid meeting or dealing with this man. May Allaah Guide you to goodness.
Allaah Knows best.