What Allaah has enjoined on women is to conceal their whole bodies from non-mahrams, including the face and hands. The clothing should be loose and not show the shape of any part of the body, and it should not provoke desire.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said:
It is permissible for a woman to sit with her husband’s brother or cousins etc., so long as she is wearing proper Islamic hijab, covering her face, hair and all of her body, because she is ‘awrah and a source of temptation, and so long as there is nothing suspect about this sitting with them, and she is not sitting alone with any one of them.
With regard to sitting alone with one of them, or in a suspicious manner, that is not permissible.
It is more important that a woman should observe hijab in front of her husband’s relatives such as his brothers, because the husband’s relatives can enter upon her and sit with her without anyone denouncing that, then that may lead to regrettable consequences. See question no. 12837.
See Fataawa al-Mar’ah Jam’a al-Musnad, p. 157.