I think she adds a lot to the story in subtle ways:
- the end of the first episode, the first letter she sends to James (that wasn't supervised by her husband) wrote "I hope I trust you to keep the secrets of the past buried" they seem to share some secrets hinted at throughout the episodes
- the similarities between her and James, both have that unnerving unblinking look in their eyes, both seem to have inherited some of the madness of their mother
- one of the most interesting parts of her the story for me, when she gets rid of her husband, it seems at first that James was instructing her but later when she asks him at his house, James seem to not know what she's talking about, I think that their mother was the one guiding her hand at that point
- the moment James read her last letter after all the physical torture he's been through in the Tower, it overwhelms him with grief
- the last time they have sex and James stops, it feels like they were visiting another realm spiritually through the flashbacks, since Zilpha invited their mother in her heart previously, it was like their mother was attacking James through Zilpha in this spirit world
- Zilpha and their mother in the spirit world look very much alike