Selectors express structural relationships between elements. When you ask for a selector for an element that is "outside" another element, you're looking for a combinator that says "this element appears outside the containing scope of this other element".
There is no such combinator.
You could conceivably select specifically the .outside sibling of .parent, but then you run into another problem that there is no parent selector for matching .parent relative to .child:hover like there is for matching .child:hover relative to .parent (that is, .parent > .child:hover).
See also: How do I select an element based on the state of another element in the page with CSS?
