بسم الله الرحمن الرحيم
Your sister should not get annoyed with such dreams. She should do taubah and istighfar extensively. In future if she experiences such dreams then should read the following dua as soon as she gets up and spit out on her left side thrice and if night remains then you should turn her side and sleep again.
أعوذ بالله من الشيطان وشر هده الرؤيا
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.