There is no sibling selector to match elements (or not) by class.
The closest selector I can think of is
.iwant:only-child
But this selector means that there cannot be any other elements besides that div class="iwant" as children of the parent div, regardless of type or class. This may fulfill your need depending on the structure of your HTML though, so it’s worth a try. If class names are a problem for you, though, then there probably isn’t much of a solution, because there isn’t an :only-of-class pseudo-class in CSS which filters by class and ignores the rest.