To determine the RadTileViewItem under the mouse (the drop target) you can use the e.OriginalSource of the drop event arguments and then get its RadTileViewItem parent with the ParentOfType<T>() extension method.
sender, Telerik.Windows.DragDrop.DragEventArgs e)
var originalSource = e.OriginalSource
var dropTarget = originalSource.ParentOfType<RadTileViewItem>();
In this case the OriginalSource property contains the element that is directly under the mouse which is a child of the hovered tile view item.
Then based on the drop target you can insert the item in your collection. However, keep in mind that when you reorder the items in the tileview with drag, they are not reordered in the original collection. This is why when you add an item after reorder its position might not be correct. To resolve this you can create a Position property in your view model and bind it two way to the Position property of the RadTileViewItems. Then when you insert the item you can also set its position.
For your convenience I prepared a sample project demonstrating this approach. Please give it a try and let me know if it helps.
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