The list of sister projects displayed comes from SiteMatrix. If the project is not a plain blue link it won't be displayed.

We can't really offer this feature for incubator wikis without significant refactoring:

• There is a single incubator wiki hosting all the test projects: we'll need to rely on filters on the title prefixes because:
  • Projects are identified by a common prefix in their titles (e.g. wb/as for the future as.wikibooks.org)
• I'm not aware of something similar to SiteMatrix that would allow us to map the different projects in incubator
• Search on incubator is probably mediocre since it's a multilanguage project (the english analysis is used)

This is just a very short list of the few problems I've identified at first glance...