Mother in law..i want to kill her with insults..what do i do now?

If you order her to cease and desist opening your mail, she won't listen (can you say passive-agressive?) There was obviously no emergency contained in the letter and her reason for opening it was ridiculous. Consider getting a post office box. If she lives with you, is there a lot, or has done this before, tell her that you will report her to the proper authorities if she does it again. Your husband - her son - should be to set some serious boundaries for Mommy Dearest.