I hate to say this, but she needs to go to the hospital or some in patient psychiatric unit if she is pulling a knife on you for closing itunes... She is sharing in their delusion they are bigger thsn jesus. Just because she is your sister does not mean you need to give her slack and to put up with her self destructive behaviors that also is becoming destructive to your environment and peace especially if she is preventing you from blocking her music out with yours. It clear harassment and would drive anyone insane..... The sounds of cars and street noises is enough to drive someone insane. I can't imagine being forced to listen to all the Beatles even the garbage. Plus in the end it will harm her more not to get help. This Beatles obsession is an obvious cry for attention and help.
Just make sure you stick with her if you initiate this even possibly just trying to talk to her first, but she just sounds unreasonable. Maybe reach out to your parents to talk to her. Either way don't put up with it. The longer you allow the behavior the worse it will get like a dig, but unlike a dog the reprocussions can be a good level more severe.
