I do "spring cleaning" every year, it is usually 3 or 4 who just get on my nerves, whether it be slow payment or something else. I've never regretted dropping any of them. Looking back, I feel I really benefitted, I now have a pretty good group of clients. Drop the son-in-law now, you won't regret it. I'm sure you do good work, there are plenty of people out there who will appreciate you. And you won't dread listening to the answering machine at night.