[BUG] Can't create a sibling about propertree HOT 1 CLOSED

Well, there are a couple issues with your approach:

1. A String is not a collection, and does not support children. The parent node's type must be either Dictionary or Array
2. The "New sibling of ..." contextual menu item adds a new sibling, not a child. Siblings exist on the same level as the item you clicked and they share a common parent node (see the outline below):

Parent
├── Child 1 (Child of Parent)
└── Child 2 (Child of Parent, and sibling of Child 1)
     └── Child 3 (Child of Child 2)

3. If you right clicked a Dictionary or Array - you would also have a "New child under..." contextual menu item that would add a new child to the right-clicked node (example below):
   
   The result of adding that new child would look like the following - which is the behavior you're looking for:
   

Given the structure and functionality of the items allowed in plists - the script is acting as it should, and I'll close this issue as it's not a bug with the script, but by design.

-CorpNewt

from propertree.