Laura,It turns out I made a bit of an error in my last post - "Cruel Sister" is on my 'Bard's Voyage' album, not 'Warrior Bard'. I must have been sleepy or otherwise out of it.
Also, I believe Pentangle recorded it a couple of different times. You can try looking for it on their album, 'A Maid That's Deep in Love' as well.
Happy Hunting! - Ian