CSS Previous Sibling Selectors - can someone explain

Hi, I found this CSS Previous Sibling Selectors and how to fake them

can someone please explain point

  1. Selecting all siblings
  • pointer events
  • not selector

there is some missing code so cant get it.

i have simple structure

  <div id="main">
          <div id="first"></div>
          <div id="second"></div>
          <div id="third"></div>

a what I want is:
when I hover on #first change #second and #third.
when I hover #second change #first and #third
when I hover #third change #first and #second

thanky for any response