family sites have to start with a blog, not a forum... when you have the habit of posting in a blog, you see if you want a second level of things.
i've had the exact same situation last year when about 25 members of my family asked me to create a site with a forum so we could meet online because we are seperated abroad the planet...
after 3 weeks - and about 500 visits, the only thing that was posted there were my own threads, as i needed to startup the site with content. i posted some albums from the family, comments about my life, questions and projects... basic things so, same as in my blog. the only echo i received was some cousins telling me that the site was boring because i was the only one posting... -- ok, my relatives are stupid -- ... that was my only answer, i closed the forum instead of spending all my time for nothing.