If e.Site is positioned at Summary, then you should be able to access the next sibling of the corresponding Device with:
e.Site.SelectSingleNode("../../following-sibling::my:Device", NamespaceManager)
Or access all of the following siblings with:
e.Site.Select("../../following-sibling::my:Device", NamespaceManager)
For preceding siblings, you would use preceding-sibling instead of following-sibling.