Get the treeNode's previous sibling node.
Do not initialize or modify it, it is created by the zTree.
JSON data object of the treeNode's previous sibling node
If have not the previous node, return null.
var treeObj = $.fn.zTree.getZTreeObj("tree");
var sNodes = treeObj.getSelectedNodes();
if (sNodes.length > 0) {
var node = sNodes[0].getPreNode();
}