That <?xml-stylesheet ...?> thing is a processing instruction so you can
read the whole XML file into an XPathDocument, then navigate to that
processing instruction and read out the target data, a regular
expression might help for that.

            XPathDocument xmlDoc = new XPathDocument(@"XMLFile1.xml");
            XPathNavigator navigator = xmlDoc.CreateNavigator();
            if (navigator.MoveToChild(XPathNodeType.ProcessingInstruction))
            {
                if (navigator.Name == "xml-stylesheet")
                {
                    string target = navigator.Value;
                    Match match = Regex.Match(target,
@"href=""([^""]+)""");
                    if (match.Success)
                    {
                        string href = match.Groups[1].Value;
                        navigator.MoveToRoot();
                        XslCompiledTransform xsltProcessor = new
XslCompiledTransform();
                        xsltProcessor.Load(href);
                        using (XmlWriter xmlWriter =
XmlWriter.Create(Console.Out, xsltProcessor.OutputSettings))
                        {
                            xsltProcessor.Transform(xmlDoc, xmlWriter);
                        }
                    }
                }
            }

In reality it might be more complex as the href URI is relative to the
XML document so you need to make sure you resolve it correctly.