How select Items?

I want my OmniOutliner automation script to select the siblings following the currently selected row.

Note:a way to get the ItemTreeNode corresponding to an Item is a sufficient answer to the problem posed below; if you can give me that you can ignore the rest.

// Given 
editor = document.editors[0]

// the currently selected ItemTreeNodes:

// another way to get the currently selected ItemTreeNodes:

// the currently selected Items:

// or, getting fancy:{return node.object})

// the siblings following the (first) selected Item

The last expression yields an Array of Items. But takes an Array of ItemTreeNodes. I don’t know how to get siblings of an ItemTreeNode (or other related ItemTreeNodes). And I don’t know how to get the ItemTreeNode corresponding to an Item. So I can’t figure out how to select siblings of a selected row.