If your sister is prone to listening at very high volumes, it is possible that she "pushed" the amp into clipping and fried the tweeter. This is especially likely to happen with a low power amp pushed too hard (as opposed to too much power).
HELP No sound from tweeter.
I just sold my Mordaunt - Shorts to my sister and after several weeks of working perfectly (as they always had) suddenly no sound whatsoever is coming from one of the tweeters. No crackling, no nothing. The posts are jumpered. What do I do? I'm not known as a tinkerer. Thanks for the advice.