If statement using jQuery ':visible' not returning boolean

I have created a fiddle here http://ift.tt/2boawi9. I am using the line

if ( !target.siblings("aside").is(":visible") ) {

to check whether the target's 'aside' sibling is not visible, but it always returns true, even after my javascript has shown the 'aside' element. Because of this my else statement will not run. You can see that clicking on a box in the fiddle will show the hidden element but when it is visible, you should be able to click on the box and hide it. What am I doing wrong?