you could do it something like this:
dim myTabId as integer = 37
dim objTabController as New DotNetNuke.Entities.Tabs.TabController()
dim objChildTab as DotNetNuke.Entities.Tabs.TabInfo = objTabController.GetTab(myTabId, PortalId, False)
if objChildTab isnot Nothing then
dim objParentTab as DotNetNuke.Entities.Tabs.Tabinfo = objTabController.GetTab(objChildTab.ParentId, PortalId, False)
end if