What is the rule as to when Newborn or Siblings section should appear in the sidebar?

The Siblings sidebar only appears in question pages, and is populated based on the tags of that question… I have no clue about the Newborn one though. Perhaps that is what is shown when there are no tags added to a question.

Ok, now I’m really interested in this and I’ll go insane until Bendrew come to enlighten us :)

This recent question has a Newborn section, but the question we are in right now is newer, so I don’t think it’s related to the age of the post.

My theory is still it being related to tags. Note that in the link above, the tag used is “religion christmas” instead of “religion” and “christmas”. Sure enough, there is only one post tagged with “religion christmas”, that very post itself. Thus there are no siblings to populate the list, thus it defaults to “Newborn” mode.

One last observation. I think it selects the results based on how precisely similar the tags are. Notice how the first sibling here is this page which has the exact same tags as your post, that being the only tag of “Fluther”. But when I click on the tag Fluther itself, I see that there have been more recent posts containing the tag, such as this one, but that one contains additional tags beyond just “Fluther”. So I’m guessing that they populate siblings based on how identical the tag set is.