MonikerRelativePathTo

[DllImport("ole32.dll", SetLastError = true)]
public static extern int MonikerRelativePathTo(
   IMoniker pmkSrc,
   IMoniker pmkDest,
   out IMoniker ppmkRelPath,
   [MarshalAs(UnmanagedType.Bool)] out bool pdwReserved
);

Microsoft documentation: Link