How to limit drag and drop capability to only sibling-level elements in angular-ui-tree?

I am developing a tree view to show list of categories using AngularJS. I used this git repository.

HTML

<div ui-tree="treeOptions" id="tree-root" data-drag-enabled="true">
    <ol ui-tree-nodes ng-model="nodes">
        <li ng-repeat="node in nodes"  
            ui-tree-node 
            data-type="top-level" 
            ng-include="'nodes_renderer.html'"></li>
    </ol>
</div>

I am able to implement drag and drop elements in the tree. I want to limit the drag and drop capability to only sibling-level elements.

I tried below but still no luck.

JS

$scope.treeOptions = {

    accept: function (sourceNodeScope, destNodesScope, destIndex) {

        if (sourceNodeScope.$parent.$id === destNodesScope.$parent.$id)
            return true;
        else
            return false;

    }
}

I cannot find much about this requirement on GitHub repository. Any help or useful link is highly appreciated.

Your logic is basically ok, but the callback you are using is not the right one. Angular-ui-tree has a beforeDrop callback that allows you to return a promise, and based on the result, will accept or reject the drop. The accept callback is actually called while you are dragging every time you cross another node. Here is a simple sample implementation:

$scope.treeOptions = {
        beforeDrop : function (e) {
          if (e.source.nodesScope.$parent.$id === e.dest.nodesScope.$parent.$id) {
            console.log("siblings, this is allowed");
            return $q.resolve();
          }
          else {
            console.log("not siblings");
            window.alert("Not siblings! Reject this drop!");
            return $q.reject();
          }
        }
     };

Here is plunkr showing a simple example: http://plnkr.co/edit/6PD6PrD3wRduOjOQzuFN