I also wanted to expand on this scenario. I will draw the three witches into this, as it is a pity to just make them yet another canon fodder in the Kreeg lair. They of course will taunt and torture the nymphs ghost, but they should also interact with the PCs...I think they will take the forms of nymphs themselves, seducing the male players to follow into death traps in order to "save their sister" I just still need the "tests" the PCs have to "pass". Maybe one will also shortly reveal her face to the only female player, to sow dissent among the party.
Ok there you could come with pixies, sewn to a necklace which is given to the Pcs by the "nymphs" as flowers, and they do not recognize it or similar stuff, but this adventure has too much of gory stuff and my players only laugh about the overuse of gore. In the end the PCs should start to hate the witches and be really happy to finally get them.
I always have Macbeth in mind when I think about the cenario.