Is it normal that my father always shakes hands with my brother in law

My bro in law has been family for over 10 years now and my father just shakes his hand as if he is just a stranger. I mean literally just about every time he shakes hands like he is just a stranger and not an in law. I think that an in law you dont have to shake hands

I mean of course you would if he accomplished something or whatever but not just to greet and shake hands especially if you are family now every single time

IMHO