I say, and with God's success: The husband's brothers are strangers to the wife, so they are not among her mahrams, and it is forbidden for her to uncover herself in front of them after they reach maturity. As for sitting with them, if she is wearing her hijab and there is no temptation, it is permissible. However, it is preferable to avoid mixing without necessity, and the danger from the husband's relatives is greater than that from strangers; therefore, the brother-in-law is considered death, referring to the husband's relative. And God knows best.