If you have any doubt about your father-in-law, don't tell him where you are getting your pot (or that you are growing it). You'd expect people who have something to loose wouldn't turn you in to the cops (or tell someone else who you'd get "in trouble" with, such as your landlord).

If someone has weak impulse control, they may just BLURT OUT something in a moment of distress which could hurt you (and/or your grow operation) without blurting it directly to the cops.

So, if your father-in-law will miss-out on free pot by disrupting your grow operation, then you'd expect (in a rational world) that he'd use his better judgment and NOT talk.

If in doubt, just keep the source of your pot to yourself.