Your classic banshee doesn't cause death so much as portend its coming. She can appear as a beautiful maid, a washer woman, or a hag. Likewise, her voice can sound lovely or loathsome. In many myths, banshees were tied to particular families and would cry out just for those families.
Sounds to me like she needs the ability to foresee death, the ability to sing, and some talent with glamours. Add in an Aspect related to being tied to a particular family if you wish, and you should be pretty well solid gold.
I'm pretty sure I recall reading a myth that involved a banshee that flew around the roof of a house, ducking in and among the eaves. So flight doesn't sound totally out of the question.
Some myths use banshees somewhat interchangeably with mermaids, which are themselves nearly interchangeable with sirens. If you wanted to stretch things a bit, you wouldn't be totally off base to add in an ability to incite emotions (probably lust).
If you wanted to go totally weirdzo, you could make your lust inspiring banshee the daughter of a Raith and a Sidhe. The family she "haunts" are the offspring of a White Court virgin who found love before lust and went on to become human. Now she spends her immortal life mourning the deaths of her cousins.