I hope you don't mind this offer of support in your business predicament with BIL. I am manager with employees under my direct supervision and can understand how prickly professional relationships can get at times. In your case, it's a family member which can make things complicated for sure.
In my home office, I have this wonderful booklet on the bookshelf titled Perfect Phrases for Dealing With Difficult People by Susan F. Benjamin. It's saved me many, many headaches. You also might want to Google Ten Tips for Dealing with A Lazy Coworker to get some ideas.
Do your homework and prepare some crib sheets. I always do when I know that a potentially difficult conversation with an employee needs to happen and they've helped me keep on point and on topic. This way I've always had the upper hand and maintaining control of the discussion.
