I dont know your parents history but the fact they were both bothered about your sister enough to get into that situation shows they do care. the world is a more dangerous place than even you realise right now and sometimes even good parents do overreact from their own fear and need to protect, and in this case the fear of what their daughter who has already hidden quite serious things from them might have more recently been up to. your sister at her age would have been aware of the fact that hiding the diary would inflame the situation,and should take some of the responsibility for what happened. if she apologises to your parents for hiding the diary and explains it was just private,