if you search google for "family tree script " you will end up with left sidebar type tree scripts .
what i do is go to your competitors sites and view their code, search the css and java script for clues to what they are using, avoid ASP as thats a nightmare.
look for extensions and credits back to thte makers
then search those terms to hunt down the code/plugin
it often helps to start an account like a free trial and then go as DEEP as you can to then look at the source code for links to what they are using.
if you find they are using joomla then search for the wordpress version.
grab the file paths you seea t the top of the source code and put it back into google, sometimes you end up gettign sent to a template for $30 !
hope that helps?
tim