Sadly, that’s not considered a section within Obsidian. That is, it’s not a logical entity within the DOM.
Not as of current, as far as I know.
We’re not looking at the siblings of elements, but the siblings of children elements to the containing divs, so you kind of have to traverse down the tree, and then up again to match something down into the next element. CSS can’t do that.
Whenever Obsidian upgrades to Chromium, version 105, we might be able to possibly do this, but not currently, as far as I know. (For more on the Chromium version, see here.