I suppose you could use GroupJive and each family be given their own Group? For now you'd need to set this up manually, but soon an auto join feature will be implemented. You could also use the Connections system and those of same family could request connections with one another. CB isn't really a directory extension, maybe review looking at mostree or something of the sort?

Is there a demo site

Yes, our demo site has GJ 2.x installed at the below link.

cbsubs1.cbdemosites.com/

where I can see how GroupJive has been implemented by others?

Not unless it's posted in the showcase boards. We do not disclose users sites unless they do so themselves.

With a couple simple workarounds, the simple user list pretty much does the job. Can I edit the css file, so, for example, the family surname (a custom field) is bold? I am enjoying the downstream possibilities of CB for this project.

Yes of course, you can edit the CSS and place in your Joomla template or CB template CSS file.

GJ comes up too late in the user experience, as a tab within each profile....i would want the start page to be a list of GJ groups, each one a family name.

That is perfectly doable, just set your sites Home page (Joomla menu link) to the GJ category where family groups would be.