Dear Confused in TX,
I thought plots this convoluted and bizarre only existed in the soap operas I don't watch. It's really unfortunate that there are actually people with such ugly personalities in the world, but of course we've seen much worse from the human race this month.
Put Everything in Perspective
Thus, my first bit of advice is, put everything in perspective. As despicable as your MIL's actions are, accept that they have occurred and prevent them from happening in the future by doing a couple of things:
1. Change the friggin' password on your voicemail.
2. Stop snooping in other people's property. They asked you to feed their cats to their fish (or something like that), not to go through their desk. You do realize how ridiculous it is for you to complain about her invading your privacy when you are guilty of precisely the same?
Make Peace with Her...
We have also seen much better aspects of human existence in recent days. Why don't you join the throngs of decent, peace-seeking individuals on this planet, offer your MIL a peace pipe (figuratively speaking, unless she's into that sort of thing literally), and see what happens. You might find this has all been a terrible, sad misunderstanding based on lack of communication (to all my fans: You didn't think I'd forget to get the C-word in there somehow, did you?).
...Unless She Just Hates You
On the other hand, it's also possible that you'll discover that your MIL is a horrible old hag who hates your guts, and that your husband was perfectly justified in breaking off contact with her, in which case you should forget she ever existed and move on with your life.
I've referred repeatedly to the mother; naturally my advice relates to the sister as well.
Pass These Lessons to Your Children
You got married very young, but have stayed together for a while. I hope your relationship is strong and will continue to be so, and I trust that you will learn lessons from these incidents and pass along the right messages to your own children.
All the best,