code: function(sel) {
        if (!this.length) {
          return this;
        }
        var el = this[0];
        return u(array.filter.call(el.parentNode.children, function(child) {
          return sel ? child !== el && u(child).is(sel) : child !== el;
        }));
      }
docs: siblings method
  get element siblings
  @param  {string} [sel] - selector to filter siblings
  @return {object}         sibling elements