Pseudo-elements cannot be targeted by sibling combinators because sibling combinators only represent element siblings, not pseudo-element siblings.
So, although the boxes generated by ::before and ::after are siblings of one another in terms of layout, for the purposes of sibling combinators they are not.
It is not possible to write a selector styling an element's ::before pseudo-element when its ::after pseudo-element is hovered. (For that matter, ::after:hover is not valid outside of Selectors 4 either, and no implementations exist.) There are hacks that make use of things like pointer-events but there is nothing that is guaranteed to work on all browsers.