The source code

    /**
     * @class Ext.Element
     */
    Ext.apply(Ext.Element.prototype, function() {
    	var GETDOM = Ext.getDom,
    		GET = Ext.get,
    		DH = Ext.DomHelper;

    	return {
    		/**
    	     * Inserts (or creates) the passed element (or DomHelper config) as a sibling of this element
    	     * @param {Mixed/Object/Array} el The id, element to insert or a DomHelper config to create and insert *or* an array of any of those.
    	     * @param {String} where (optional) 'before' or 'after' defaults to before
    	     * @param {Boolean} returnDom (optional) True to return the raw DOM element instead of Ext.Element
    	     * @return {Ext.Element} The inserted Element. If an array is passed, the last inserted element is returned.
    	     */
    	    insertSibling: function(el, where, returnDom){
    	        var me = this,
    	        	rt,
                    isAfter = (where || 'before').toLowerCase() == 'after',
                    insertEl;

    	        if(Ext.isArray(el)){
                    insertEl = me;
    	            Ext.each(el, function(e) {
    		            rt = Ext.fly(insertEl, '_internal').insertSibling(e, where, returnDom);
                        if(isAfter){
                            insertEl = rt;
                        }
    	            });
    	            return rt;
    	        }

    	        el = el || {};

                if(el.nodeType || el.dom){
                    rt = me.dom.parentNode.insertBefore(GETDOM(el), isAfter ? me.dom.nextSibling : me.dom);
                    if (!returnDom) {
                        rt = GET(rt);
                    }
                }else{
                    if (isAfter && !me.dom.nextSibling) {
                        rt = DH.append(me.dom.parentNode, el, !returnDom);
                    } else {
                        rt = DH[isAfter ? 'insertAfter' : 'insertBefore'](me.dom, el, !returnDom);
                    }
                }
    	        return rt;
    	    }
        };
    }());