Whether or not you can be forced to pay dues for the time you were inactive depends entirely on what you agreed to as a member of the sorority. You'd need to look at any sorority rules or regulations, any paperwork you signed, etc. and see what your obligations are. There is no hard-and-fast or general rule for situations like this--it is equally legal to bill people for inactive time or to not bill them for while thhey were inactive. The question of fees depends on what you agreed to. If there's ambiguity and you think they might sue or send you to collections, you may wish to ask yourself  if the money you'd potentially save is worth the costs, monetary, inconveniencee, time, etc. of fighting it. Good luck.