My first reaction is to say that the friend of your brother, so long as you have shadowed for him as well, would be OK. Just because you know him personally doesn't automatically disqualify him. I think many people may know their optometrists (or those who write the LOR's) in more than just a shadowing capacity. Just to throw out a few hypotheticals, you may know their optometrist because he is also a member of your church, or because your parents are in a business or volunteer association together. I think the mention in the FAQ's about not allowing letter from family friends says that so someone's personal family friend doesn't write a letter about how great you are. Yes you know this optometrist through your brother, but that is just networking, and not, in my mind, what this qualifier is about. Plus, in the LOR process you nor anyone else will see what this optometrist says, so there shouldn't necessarily be a bias just because he knows your brother. In short, it's great that you have connections to this optometrist through your brother. So long as you shadow him in a professional sense, I think he is a candidate for writing your letter.