Can a Son Sleep Together with His Mother in Islam?
Question

Is it allowed for a boy to sleep in the same bed with his mother or sister after he has reached the age of puberty?
Praise be to Allah.The Standing Committee for Academic Research and Issuing Fatwas said:
It is not allowed for male children, if they have reached the age of puberty or are ten years old or more, to sleep with their mothers or sisters in the same bedding or mattress. This safeguards chastity and keeps the person away from temptation. It also closes the door to evil.
The Prophet (peace and blessings of Allah be upon him) ordered that the children be separated in their bedding when they reach the age of ten. He said, “Order your children to pray when they are seven years old. And spank them [to exhort them] to do it by the age of ten and separate them in their bedding.”  (Narrated by Abu Dawud and Ahmad. Al-Albani has classed it sound in Sahih Al-Jami`, vol. 2, p. 1021)
Those who are not approaching the age of puberty still must ask permission to enter upon their parents at three times during the day. These are the times in which one is more likely to be taking off his clothing and exposing the parts that are usually covered. This has been stressed by them being called times of privacy. Allah Says (interpretation of the meaning):
{O you who believe! Let your slaves and those among you who have not come to the age of puberty ask permission [before they come to your presence] on three occasions: before morning prayer, and while you put off your clothes for the noonday [rest] and after the `Isha’ [Night] Prayer. [These] three times are times of privacy for you. Other than those times there is no sin for you or for them to move about, attending to each other. Thus Allah makes clear His signs to you. And Allah is All-Knowing, All-Wise.} (An-Nur 24:58)
However, those who are past the age of puberty must seek permission to enter at all times of the day. Allah Says, (interpretation of the meaning):
{And when the children among you reach the age of puberty, then let them (also) ask for permission, as those senior to them (in age ask permission). Thus Allah makes clear His signs for you. And Allah is Al-Knowing A-Wise.} (An-Nur 24:59)
All of this is to avoid any kinds of problems and temptations and to safeguard honors. It also brings an end to the means that lead to evil.
As for the child who is less than ten years old, it is permissible for him to sleep with his mother and sister in their bedding if there is some need to look after him and if there is no fear of temptation. They may also all sleep in the same area, in their own bedding, if they are of the age of puberty if there is no fear of temptation.
For more details, please see the following answers:
Can a man sleep in the same room as his sister?
Is it permissible for her to sleep next to her sister on one bed?
What is the appropriate age to teach children sex education?
And Allah knows best.