I would go with jQuery or some other Javascript Framework, the selectors just rock and NOT class XY is rather easy to achieve.
As Pekka pointed out I am not sure what brothers you want to target. getElementsByClassName() is implemented by almost all browsers (you know which one doesn't work, don't you?).
I found a rather nifty solution on devshed to also make it work in IE:
onload=function(){
if (document.getElementsByClassName == undefined) {
    document.getElementsByClassName = function(className)
    {
        var hasClassName = new RegExp("(?:^|\\s)" + className + "(?:$|\\s)");
        var allElements = document.getElementsByTagName("*");
        var results = [];
        var element;
        for (var i = 0; (element = allElements[i]) != null; i++) {
            var elementClass = element.className;
            if (elementClass && elementClass.indexOf(className) != -1 && hasClassName.test(elementClass))
                results.push(element);
        }
        return results;
    }
}
}
All you need to do now is to iterate through all your div classes and negate the one you DON'T want.