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 is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Islam encourages a wife to obey her husband as long as there is no disobedience to Allaah. According to the teachings of Islam obedience to a husband is among the great acts of worship and a path to
The Prophet said: "If I were to order someone to prostrate to other than Allaah, I would have ordered the wives to prostrate to their husbands for the great right Allaah made due to them from their wives." [Abu Daawood]
The Prophet said: "A woman who performs her five prescribed daily prayers, fasts the month of Ramadan, guards her chastity and obeys her husband (in good matters), she will be addressed (on the Day of Resurrection): Enter Paradise from any gate you like." [Ibn Hibbaan]
Therefore, you have to avoid sitting with your brother in-law in obedience to your husband especially since he is commanding a good act. Disobeying him on the plea of his failure to observe the same rulings is not allowed. Your husband's disobedience in this matter does not justify you to displease Allaah by violating His right, or to consider his good advice as injustice. Instead, you or someone else should advise him to fear Allaah and avoid sitting with marriageable women without necessity and without observing the rulings of Islam in this regard. In fact, he is more entitled to implement those rulings which he orders others to follow and refrain from the evil which he advises others to avoid. Otherwise, he resembles those who order others to good but they themselves fail to do so as Allaah says (which means): {Do you order righteousness of the people and forget yourselves while you recite the Scripture? Then will you not reason?}[Qur'an 2:44].
We would like to draw your attention that you should not use insulting expressions to describe your husband, like "so stupid" since Allaah made his rights great upon you.
Allaah knows best.