Rescue Remedy goes in the water, so that is an idea.

You could also try Feliway. Oscar has to be on Feliway twice a day...I got him an inexpensive totally stretchy collar - and I gently squirt some of the liquid into the fabric - then put the collar on him. Works like a charm.

Would her being in a carrier with another of the cats help?

Poor Sparkles! I hope you find something to help her.