what do you do with family "spares?"

For me it depends on the family and the spares. Sometimes they stick around and help with skill points (legacy style) sometimes I get them relationships and the beginnings of families then move the new little families into their own homes. I try to give them a good start, but in the end they usually just run around the world unless I am feeling particularly mean and 'cull' them so that future generations aren't marrying distant family members.

I'll admit that I've killed off a few of them for tombstones for the family graveyards too. Though, I typically send the spirits off into the netherworld so that the tombstones are just empty decoration with names on them.