Q: Firstly: Is it permissible for a woman to expose her face in the presence of her husband's brothers or cousins? Secondly:
Is it permissible for a boy to sleep with his mother or sister when he becomes of age?
The husband's brothers and cousins are not Mahrams (spouse or unmarriageable relatives) to one's wife, consequently, it is not permissible for a wife to expose any of her body which should be covered up before them even if they are righteous and trustworthy. Allah confined the types of people who are permitted to see women's adornment in His Saying:
...and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, head-cover, apron, etc.), and to draw their veils all over Juyûbihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands, or their fathers, or their husband’s fathers, or their sons, or their husband’s sons, or their brothers or their brother’s sons, or their sister’s sons, or their (Muslim) women (i.e. their sisters in Islâm), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of feminine sex.
The Prophet (peace be upon him) said:
Milk relationships are equal to blood relationships in rendering marriage unlawful, i.e. whatever is prohibited due to blood relationships is also prohibited by milk relationships.
The husband's brothers and cousins are not from these types. Allah does not differentiate between a righteous person and another to protect people's honors and ward off evil and corruption. (Part No. 17; Page No. 408) It is authentically reported in Sahih (authentic) Hadith:
That the Prophet (peace be upon him) was asked about Al-Hamw and he said: Al-Hamw is death.
Hamw is: the husband's brother and marriageable men. Therefore, a Muslim should maintain his religion and protect his honor.
It is not permissible for male children when they reach puberty or become ten years old to sleep with their mothers or sisters in their bed room or on one bed to protect them against seduction and for warding off evil. The Prophet (peace be upon him) instructed his Companions to separate between children in sleeping when they reach ten years. He said:
Command your children to pray when they become seven years old, and beat them for (neglecting) it when they become ten years old; and arrange their beds (to sleep) separately.
Allah (Exalted be He) commands those who have not reached puberty to seek permission before entering houses in the three times of rest for fear that some people may expose parts of their bodies at these times. He confirms this by naming them `Awrahs (parts of the body which must be covered). He (Exalted be He) says:
O you who believe! Let your slaves and slave-girls, and those among you who have not come to the age of puberty ask your permission (before they come to your presence) on three occasions: before Fajr (morning) Salât (prayer), and while you put off your clothes for the noonday (rest), and after the ‘Ishâ’ (late-night) Salât (prayer). (These) three times are of privacy for you; other than these times there is no sin on you or on them to move about, attending to each other. Thus Allâh makes clear the Ayât (the Verses of this Qur’ân, showing proofs for the legal aspects of permission for visits) to you. And Allâh is All-Knowing, All-Wise.
(Part No. 17; Page No. 409) He also commands those who have reached puberty to seek permission at all times upon entering homes. Allah (Exalted be He) says:
And when the children among you come to puberty, then let them (also) ask for permission, as those senior to them (in age). Thus Allâh makes clear His Ayât (Commandments and legal obligations) for you. And Allâh is All-Knowing, All-Wise.
This is to ward off seduction, protect people's honors and eliminate means of evil. As for children under the age of ten, it is permissible for them to sleep with their mother and sister in their bed because of their need for care and for removing hardships when fearing no seduction. However, in case of fearing no seduction, it is permissible for all of them to sleep together even if they are adults in one place but everyone should sleep on a separate bed. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.