My dentist LOR was from a brother in-law. He didn't state anywhere in the letter that we were relatives but anytime an interviewer asked about shadowing with him, I was honest and explained our relationship. They didn't seem to mind. Besides, the dentist LOR probably isn't real valuable. I'm sure schools know anyone could get one from their family dentist without putting a lot of effort into it. Get it from your in-law though b/c it will be of better quality.