Well it would seem it is just pre made dead, I just loaded my townie dynasty at the point before I moved gen 4 to Sunset Valley and picked up all his relatives before going and everybody showed up in the family tree, founders Aiden and Hannah Jones, their five kids, first heir and spouse plus descendants, also her sister's spouse and descendants.
So entire tree intact apart from Hannah's mother and the rest of the Remingtons, didn't think to pick those up actually, but it's interesting, I think if you had family space you could possibly make them playable then send them back to the netherworld on arrival, that might keep pre made dead sims in the tree.
One really weird thing is his IF doll came too when it was sold with his previous house before marrying, it did that when I moved them before, those dolls are odd.
My file went from 1.01GB to 917MB not bad I guess, still far too big for my liking though