Your husband is wrong in insisting that his brother who is an unmarried adult stays with you in the same apartment on a regular basis.  The Prophet (peace be upon him) warned us against such actions: He said, “In-laws are death!” (Al-Bukhari).

 

In other words, even as isolation involving two strangers is a grave sin, exposing ourselves to isolation with in-laws is far worse; it could be as severe as death!

 

The Prophet (peace be upon him) also told us to stay away from the doubtful for it may inadvertently lead to that which is clearly haram; it is like cattle allowed to graze near the protected territory; by doing so, it may end up trespassing and grazing there!

Therefore, he stated: “Whoever stays clear of the doubtful will guard his honor and religion.”  (Muslim)  But as for those who do practice what is doubtful may end up hurting their honor and religion!

You, therefore, have the right to insist that the brother discontinues this practice.

If he does not, you have the right to part ways as he is playing with fire.