Please pay attention that there is no tag Item inside tag source_id. Thus getPropertyItem is not applicable.

If you try

IOM.Item itemProp = result.getPropertyItem("related_id");

textBox1.Text = itemProp.dom.InnerXml;

this should work.

Since you know ItemType id you may request the ItemType. For example:

string id = result.getProperty("source_id");
IOM.Item it = Inn.newItem("ItemType", "get");
it.setID(id);
it = it.apply();

Also I noticed that you specified incorrect type name. That should be RelationshipType and not relationshipType.