I have three of my cousins on Facebook. I'm really, really close with two of them, so I view them as just another friend. The third cousin I don't really talk to in person or on the site.
My dad has a Facebook. I blocked him. We've never gotten along.
I think that adding family to FB is fine. If you don't want to shut down that means of communication, but you don't want them to see certain things, two words: privacy settings.