Better to ask your uncle. Money spent with a professional might be better spent than your free site.
C++ wasn't known as a language for web development when I was writing it.
You'll need a web-based user interface technology. HTML, CSS, JavaScript are the standards there. Flex might be good in this case, because visual effects can be so helpful.
You'll need a CMS or persistent store of some kind. What do you know about relational databases?
Are you ready for the maintenance burden as well? It's likely that you won't be able to walk away if the site goes live. There will be bugs, fixes, upgrades, enhancements. Have you signed on for that as well?
It's a big problem. You need to make sure that you're doing your uncle and his business a favor by offering your labor. It's not a trivial task.
Is buying a better idea? Do some research first:
http://www.google.com/search?rlz=1C1GGLS_enUS291US323&aq=f&sourceid=chrome&ie=UTF-8&q=real+estate+software