css3 - CSS: how to select parent's sibling

I have the following HTML in my page.

    

        
        Show Details
    
    
content ...
    
content ...
    
content ...
    
content ...
    
content ...

I am unable to change this HTML. I have hidden all LI's with the exception of the first by using the following CSS

ul#detailsList li:nth-child(1n+2) {
    display:none;
}

So far so good. What I want to do now is to show those hidden LI's when the the checkbox is ticked, using pure CSS. My best attempt so far is

ul#detailsList li input#showCheckbox:checked + li {
    display:block;
}

Obviously this doesn't work, as the + li will only select LI's immediately after the checkbox (i.e. siblings), not siblings of the parent.

Is this even possible?