On a CSS hover event, change another div's styling. Without elements between:

#a:hover + #b {
    background: #ccc
}
...
<div id="a">Div A</div>
<div id="b">Div B</div>

On a CSS hover event, change another div's styling. With elements between:

#a:hover ~ #b {
    background: #ccc
}
...
<div id="a">Div A</div>
<div>random other elements</div>
<div>random other elements</div>
<div>random other elements</div>
<div id="b">Div B</div>