[brianogilvie]

A sibling would be an action at the same hierarchical level as another action or action group. A child is an action at a lower level. An aunt, meanwhile, is an action at a higher level. (Why not uncle?)

OmniFocus has several ways of creating projects, action groups, and actions. The Outlining commands (Edit >*Outlining) use traditional outline-speak (child, aunt) for these. Adding a child to a project creates a new action in the project, while adding a child to an action turns the original action into an action group with a new action in it. Adding an aunt to an action group creates a new action at the same level as the group, while adding an aunt to an action that is not part of an action group creates a new project.

You can use OmniFocus productively without worrying about the kinship system of outlines, though! :-)