Here's a little something I picked up from an old movie last night. I'm not saying it will necessarily come in handy for you; indeed, I hope it won't. But imagine you did find yourself in this situation and I'd never warned you? I'd never forgive myself.
If you come to suspect that your fiance has killed two people (or three, depending on how you consider the unborn child of your sister), do not share these thoughts with him while the two of you are ALONE at a MASSIVE QUARRY.