[J_is_my_initial]

Don't really know what you should do, but I feel for your situation. I had a lot of issues with my mother in law (def not her favorite person) but to actually call the law on her would cause a lot of problems. Talk it over with your fiancee. See what he thinks you should do.
Keep in mind the best thing to do imho would be to call it in to possibly help the other animals. But it would be a call only you can make in the end no matter our opinions cause you have to live with the repercussions of it.