Text or email ALL your sister-in-law's relatives and friends and tell them she cannot be trusted to not snoop through their personal belongings. Tell them not to leave their medical info, credit card statements, medications or other personal info any place that isn't physically locked with an actual key.
Once knew of a couple who had a bookstudy in their home. A person was using their restroom and snooping through their medicine cabinet every time they went. The couple put marbles in their medicine cabinet. The next time the snooper open their cabinet, the sound of marbles falling into the sink could be clearly heard to all attending the study. The couple were in hysterics, and the snooper was caught and never came to their bookstudy again.