Is it permissible for a married woman to go out with her sister and her husband, or to sit with them in the same gathering, while covering her face and wearing an abaya, believing that her sister's husband is like a brother to her and that this is part of their customs?

It is forbidden for a non-mahram man to do with a wife what is forbidden for other non-mahrams, such as looking at her 'awrah (private parts), being in khalwa (seclusion) with her, or touching her body. What some societies have become accustomed to, regarding women mixing with their non-mahram relatives, is not permissible. It is permissible to speak with them out of necessity, to the extent of that necessity, while adhering to the etiquette of speech that Allah commanded in His saying: "So do not be soft in speech [to men], lest he in whose heart is disease should covet, but speak with appropriate speech." It is permissible for a woman to go out to the market with her sister and her husband, provided there is no khalwa and no fear of temptation (fitnah). It is permissible for her to sit with him in one gathering from behind a حجاب (screen or partition) if the Shariah guidelines are observed, such as the absence of khalwa, the absence of suspicion, the absence of softness in speech, and for a [valid] need.