Answer
(Fatwa: 980/980/M=1432)
According to Shariah it is forbidden for two males or two females to sleep or lay down together in a single blanket or sheet when both of them are naked and they have no cloth between them:
نهى رسول الله صلى الله عليه وسلم عن مكامعة الرجل الرجل بغير شعار ومكامعة المرأة المرأة بغير شعار الخ (مشكوة).
Also it is mentioned in the annotation of Mishkah:
عن مكامعة الرجل أى مضاجعة الرجل صاحبه في ثوب واحد لا حاجز بينهما بأن يكونا عاريين . (حاشيه مشكوة).
However, if there is cloth etc between both of them, then though it is not unlawful sleeping two persons in the same blanket but it is better for them to sleep separately even if they are mother and daughter.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband