An updated Apple Support Document details when label printing does (and does not) include related names on a label in OS 10.4 -10.5 and 10.6.

- http://support.apple.com/kb/HT3952

The short of it is they made a nice feature that tried too hard to be smart.

In 10.4-10.5:
Address Book will NOT print a related name (ID'd as spouse, child, etc,) on a label IF another Address Book card has the same first name with a different address.

You can force it to work despite the above if you make a card for each related person that includes matching addresses.

In 10.6:
Address Book does not print related names in labels.

The work arounds are:
- Manually: use "John & Jane" or whatever you want for the first name
- Hack: add the 10.5 Address Book plist file to the 10.6 version (search for ABRelatedNamesConfig.plist)

but honestly I think the feature's logic limits don't make it worth the trouble.

A shame. Because if it was "dumber" and just included related names, or a toggle at print time to do so, it would be fine.