Is there any method to get the index of father and son by the idTree?
There is no ready method, but you can get parent index the next way
(i.e. with selected item var selected = tree.getSelectedItemId() ):
var parentIndex = tree.getIndexById(tree.getParentId(selected))
You need to use method getAllSubItems(selected), then to apply method getIndexById to every sub item (or a special one you need)