If you want to achieve rounded corners with siblings, you can specify which corners should have this feature. Take a look at this helpful page for more information on how to do it. Essentially, it involves using the CSS property:
border-top-left-radius: 10px 5px;
.
Here's an example:
<div id = "container">
<div></div>
<div></div>
</div>
For styling in CSS:
#container div:first-child
{
border-top-left-radius: 10px;
border-bottom-left-radius: 10px;
background-color: blue;
}
#container div:last-child
{
border-radius: 10px;
background-color: rgba(0,0,0,0);
/* Same color as the sibling div and a distance of the radius + the separation */
box-shadow: -12px 0 blue;
}
You may want to give this a try. This method works well. You can also check out the updated jsfiddle demo for some additional aesthetic tweaks based on your image.
UPDATE: I made adjustments to ensure the correct side has the same color as the original border. Check out the revised jsfiddle here and refer to the modified code above.