Great suggestion! Thank you.
Solved my problem, but it is actually: System.Web.SiteMap.CurrentNode.ParentNode.Url
My solution worked like this: Master page Control not shown for Top level (Home), show children (no override) for pages parented by home page, and show sibling pages for anything deeper than that.
/* Conditional formating for Secondary Nav to show siblings */
if (Children != null && System.Web.SiteMap.CurrentNode.ParentNode !=
null) // make sure Control exists and Page has a parent
string parentURL = System.Web.SiteMap.CurrentNode.ParentNode.Url;
if (parentURL != "/") // if parent not top level (Home)
// Show siblings - Parent page is not root, this is 3rd level or lower
"/" + parentURL;