root = QgsProject.instance().layerTreeRoot()
group = root.findGroup('Main group')

vlayer = QgsVectorLayer('LineString?crs=epsg:27700', 'vlayer', 'memory')
QgsMapLayerRegistry.instance().addMapLayer(vlayer, False)
group.insertChildNode(-1, QgsLayerTreeLayer(vlayer))

group.insertChildNode(-1, QgsLayerTreeLayer(vlayer))

group.insertChildNode(0, QgsLayerTreeLayer(vlayer))

bridge = iface.layerTreeCanvasBridge()
order = bridge.customLayerOrder()
order.insert( 0, order.pop( order.index( vlayer.id() ) ) ) # vlayer to the top
bridge.setCustomLayerOrder( order )

def rearrange( layers ):
    order = iface.layerTreeCanvasBridge().customLayerOrder()
    if order: # If no existing layers, no need to do anything
       for layer in layers: # How many layers we need to move
          order.insert( 0, order.pop() ) # Last layer to first position
    iface.layerTreeCanvasBridge().setCustomLayerOrder( order )

QgsMapLayerRegistry.instance().legendLayersAdded.connect( rearrange )