Show siblings on children pages (not children) in menu

Hello! I’ve got a page structure like this:

– Apples
– Red
– Green
– Blue
– Orange
– Bananas

On Apples, I want to show the first-level children as subnav (Red and Green).

On Red, I want to show its siblings (Green). It should not show its children (Blue and Orange).

Is there a way to specify this? Essentially, if it is top-level, show children. If it’s a child page, show siblings (do not show additional children).