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 (صلّى اللَّهُ عليه وسلّم) 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.