The username of the half-sibling on the online DNA site starts with bolitarfan. Myron and Mickey Bolitar are the main characters in many other books by Harlan Coben.
British Army Berrets are worn in a specific manner. Tye badge is over the left eye and the berret shaped to sit down on one side. No military person would wear their beret like a hat on top of their head. It's a huge giveaway.