Saaiduna Abu Said Khudri Radiallahu Anhu, reported from his father: The Prophet of Allah Sallallahu Alahi Wasalam said: “A man should not see the private parts of another man, and a woman should not see the private parts of another woman, and a man should not lie with another man under one covering, and a woman should not lie with another woman under one covering” (Sunan Abu Dawud p.201 v.2, Sahih Muslim and Musnad Ahmad)
The explanation of the aforementioned hadith is that a man is not allowed to sleep with another man or a woman is not allowed to sleep with another woman under one sheet or duvet without a barrier, i.e. both being naked. If however, they are covered and are sleeping under one duvet then it will be permissible, althoughit is preferable to refrain from due to the danger of mischief and temptation. (Durr Mandhuud p.119 v.6)
The aforementioned ruling is applicable whether the two women are sisters or not.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham.