This seems like a really awkward way to implement this. A more elegant way to support this would be negative ZIndex values for Sibling. That’s not how ZIndex works right now, but that’s how it needs to work for Sibling to be useful.
Conditionally treating the parent as a sibling isn’t very intuitive on its own. This would work better as an additional ZIndexBehavior, such as “SiblingAndParent”.
It’s just more work, and an extra layer of GUI objects that you need to go through when scripting.
Also as a UI designer I am thoroughly sick of making containers for everything lol
Using a constraint? Container.
Using some sort of special background design? Container.
Organising your work? Container.
Using sibling ZIndex behavior? Container.