I appreciate the comments. It certainly helps to get outside perspective on something that I can't share with friends. My long time partner knows, but I need more outside perspective to make sure that I am thinking clearly about this issue.
As a hypothetical, if my brother were a pedophile, arrested and slapped on the wrist in a foreign country, I would already have the website up publishing his name. Family loyalty is just balderdash when a family member is doing something terribly wrong. My brother is not a pedophile thank goodness, so (fortunately) it is not as easy for me to decide what to do.
I think a lot hinges on how much damage (if any) one thinks a mid-40s man who pursues 16-19 year old girls is causing. I am much more aligned with Tim Wood's way of thinking:
A sexually experienced (100+) middle aged man who targets 16-19-year-olds is like a ranked professional boxer who fights high schools boys. Fair fight? Admirable? Healthy and beneficial to all concerned? Or just maybe something wrong. — tim wood
Of course, that's my own and Tim's perspective and I don't expect everyone to share it. But if I only broadcast the facts (my brother sleeps with 16-19 year olds) and let people decide whether they approve, disapprove or are indifferent, I am not destroying his life if people don't care. My brother thinks he does nothing wrong. He says that his friends in the foreign country wouldn't think it a big deal if he had a 16 year old girl friend. If no one but a few people care, then publishing this information would be as harmless as publishing that someone drinks during a prohibition. (An imperfect analogy, but my point is that if few people care, then there is no social sanction and my brother can continue on with his ways.) And if people do care and don't want that behavior within their community and my brother's life is severely limited by publishing his history, then isn't that something good?
The question that I have is whether just because something is legal, should there be no social sanction? Should a mid-40s male pursuing 16 year old girls be tolerated within a community, even if legal? I think not, but perhaps in 100 years, such behavior won't even raise an eyebrow.
I think there is a bit of confusion from others (such as Hanover) about my dilemma. If my brother did sleep with a 14 year old girl, I would go to the police and I wouldn't be asking for perspective from this forum. So to clarify my dilemma, suppose the age of my friend's daughter were 16. Should I then tell my friend about my brother's preference for girls that age? And crucially, if you think I should tell my friend, then why shouldn't I broadcast my brother's sexual choices to the world? My friend's 16 year old daughter is not inherently more valuable than other 16 year old girls.
The difficulty I have is that in the above situation, I would be inclined to tell my friend about my brother if his daughter were 16 but I am much LESS inclined to make a website broadcasting to the world my brother's sexual preference for 16-19 year olds. But I am not sure whether there is a principled or reasoned difference between my two inclinations.