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 (most scholars hold covering the face is a highly emphasized sunnah but not obligatory), 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.
Ref: Fataawa al-Mar’ah Jam’a al-Musnad, p. 157.
It was narrated from ‘Uqbah ibn ‘Aamir that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Beware of entering upon women.” A man from among the Ansaar said, “O Messenger of Allah, what about the in-laws?” He said, “The in-law is death.” (Narrated by al-Bukhari, 4934; Muslim, 2172)
Al-Nawawi said: The scholars of Arabic language are agreed that al-hamu (translated here as “in-law”) refers to the relatives of a woman’s husband, such as his father, paternal uncle, brother, brother’s son (nephew), cousin (son of paternal uncle), etc. Akhtaan (sing. khatan) refers to the relatives of a man’s wife, and ashaar (sing. suhr) refers to both.
Ref: Sharh Muslim, 14/154