Is it permissible for her to sleep next to her sister on one bed?
The saheeh Sunnah indicates that it is obligatory to separate children in their beds once they reach the age of 10. Abu Dawood (418) narrated – in a report which was classed as saheeh by al-Albaani – from ‘Amr ibn Shu’ayb from his father from his grandfather who said: the Messenger of Allaah (blessings and peace of Allaah be upon him) said: “Tell your children to pray when they are seven years old and smack them if they do not do it when they are 10 years old, and separate them in their beds.”
Al-Daaraqutni and al-Haakim narrated from Sabrah ibn Ma’bad that the Prophet (blessings and peace of Allaah be upon him) said: “When your children reach the age of seven, separate their beds and when they reach the age of 10 years, smack them if they do not pray.” This hadeeth was classed as saheeh by al-Albaani in Saheeh al-Jaami’, no. 418.
This includes males with males, females with females and males and females.
The scholars interpreted separating them in their beds as meaning two things:
The first is separating their beds, which is the apparent meaning of the second hadeeth.
The second is not letting them sleep dressed lightly on the same bed, but if they sleep in their clothes without touching, that is permissible so long as there is no risk of fitnah.
Zakariyya al-Ansaari (may Allaah have mercy on him) said: Separating them in their beds may be done in two ways: if each child has his own bed, or if they are in one bed but separated and not touching one another. We should be content with the second meaning, because there is no evidence for interpreting the hadeeth according to the first meaning on its own. Al-Zarkashi said: Interpreting it on that basis is the apparent meaning; indeed it is the correct meaning because of the first hadeeth, “and separate their beds.” And it is also supported by the reason behind it, which is the fear of wrong action. End quote from Asna al-Mataalib (3/113).
And it says in Kashshaaf al-Qinaa’ (5/18): When siblings reach the age of 10 years, whether they are males or females, or females and males, their guardian should separate them in their beds, so he should give each one of them a bed of his own, because the Prophet (blessings and peace of Allaah be upon him) said: “And separate them in their beds”, i.e., when they have been sleeping dressed lightly as it says in al-Mustaw’ab wa’l-Ri’aayah. End quote.’’
Based on that, the basic principle is that each girl should have her own bed, and not share with her teenage or little sister. But if that is not possible, and they need to sleep altogether in one bed, or two of them need to sleep in one bed, there is nothing wrong with that, so long as each one of them has her own cover or blanket.
Al-Haafiz Ibn Hajar said concerning a group of people sleeping in one bed: It has been proven by other ways that it is essential that they do not share one blanket. End quote from Fath al-Baari (7/204).
We ask Allaah to help and guide you.
And Allaah knows best.