I would recommend placing a personal ad in Anchorage newspapers. Unless your half-brother & other relatives are themselves interested in genealogy, they are more likely to learn of your query through ads in general circulation newspapers (or through friends who read the ads) than on a genealogical website.
The Anchorage Daily News <http://www.adn.com/>
;, besides being the largest newspaper in the state, also has a statewide circulation. There's a decent listing, with links, of other Alaska newspapers at <http://justice.uaa.alaska.edu/refrenz/news.html>
;. The Fairbanks Daily News-Miner and the Juneau Empire are probably the next-largest newspapers, and also have daily circulation.
Best of luck in finding your brother.