It states she can dual-cast as well. The lightning is not something only limited to her. Its ethereal because she knows that it would destroy whatever metaphysical glue was holding it together. Crystalle could have casted this herself. Ishtalle was simply more active.

Edit: In simple terms, the lightning was a spell cooked up by ishtalle based on information gathered through interactions with the staff, and not wholly based upon her Adaptive Edge ability.