I have not heard of this happening before...
If they are not hostile, then Firewall probably wouldn't help, and you need a hostile target around to use Splitting Arrows...
Try emailing support@larian.com. They may be able to reset Xanlosch and Isabelle's status with your save, or you could email them first and see if they have any suggestions or instructions.
With the last Xbox firmware update (through Xbox Live or copied to a CD/DVD or USB flash drive) you can copy saves to a flash drive, and with the right software can copy it to your PC. I haven't gotten around to backing up any saves yet, but the software apparently required is called Xtaf Xplorer.
