I am getting really upset, guys. I'm in the Dark Side and Forest of Lost Souls. I approach Jet's now bansidhe sister and my party definitely does her damage (as, of course, we're taking damage). Problem is when the sister casts 'ethereal', she up and disappears never to be seen again, ever. I have searched and searched, to no avail. And to go to the next quest, I must free her spirit.....

Any suggestions? Is this an individual glitch?

I think you may have freed her from her ghostly state and the quest is completed. Try posting this question at www.dsotsc.com forum and see what they say.

no,the same thing happened to me.i couldn't progress.go back to a prior save and try it again.i think it took me two attempts to get her.

FYI, I went to the DSoSC site and discovered that my trouble isn't a glitch. The key to dispatching the sister has to do with 'aura cleansing' and a multitude of missiles before she can wail like the bansidhe she is! Otherwise, (now here's a duh!) she can be damnably hard to find again and the quest nearly impossible to finish (impossible unless you *do* find her and kill her)