A mature child sleeping on the same bed with his mother and sister – Permanent Committee

Q: Is it permissible for a boy to sleep with his mother and his sister if he has reached the age of puberty?

A: It is not permissible for boys who have reached the age of puberty or ten years of age to sleep with their mothers or their sisters in the same bed, out of protection of honors, to keep away from Fitnah (temptation) and block the means leading to sin. The Prophet (peace be upon him) asked us to separate the beds of boys and girls when they reach the age of ten in his saying:

Command your children to offer Salah (Prayer) when they become seven years old; beat them for (neglecting) it when they become ten years old; and arrange their beds (to sleep) separately.

[1] Abu Dawud, Sunan, Book on Salah, no. 495; and Ahmad, Musnad, vol. 2, p. 187.

As for those who have not reached the age of puberty yet, they are asked to take permission when entering the rooms three times when people put on light clothes and `Awrah (parts of the body that must be covered in public) most probably could be revealed.

Allah (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.  [Surah Al-Nur, 24: 58]

Those who have reached the age of puberty are required to take permission at all times.

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. [Surah Al-Nur, 24: 59]

All this is for the purpose of keeping away Fitnah, protecting honors and eliminating the means leading to evil.

As for boys who are less than ten years old, it is permissible for them to sleep with their mothers and sisters in their beds, as they need care and to remove hardship. However, if there is no fear of Fitnah, it is permissible for them to sleep in the same room, even if they are adults, but each one on a separate bed.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.

The Permanent Committee for Scholarly Research and Ifta’
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Q 12: A man has many children and lives with his family in a small room that hardly accommodates them. What should he do in order not to contradict the order of the Messenger of Allah that children should be separated in beds at certain age?

A: Having separate beds for children is Wajib (obligatory) according to one’s ability. The Muslim should strive hard to fulfill that. Allah (Exalted be He) said, So keep your duty to Allâh and fear Him as much as you can The Prophet (peace be upon him) said, When I command you to do anything, do of it as much as you possibly can.

May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.
http://alifta.com/Fatawa/FatawaChapters.aspx?languagename=en&View=Page&PageID=10375&PageNo=1&BookID=7