Is Zilpha Horace Delaney's or Salish's daughter?

At one point during the course of Taboo I learned that Zilpha isn't actually James's sister, but only his half-sister. But I don't actually recall it being said from whose side of the parentage, although I doubt it was kept unclarified and I probably just missed it (but am willing to be surprised).

On the one hand it makes sense for her to be Horace Delaney's daughter, since she was expected to inherit his stuff, especially Nootka Sound, which she already negotiated to sell to the East India Company. But on the other hand, seeing how James's relation to his mother Salish is much more "mystical" and she is generally the more "interesting" or significant parent from a plot perspective, it would make more sense for Zilpha to be Salish's daughter, especially with regards to James's and Zilpha's incestuous relationship and the "wilder" nature sleeping in Zilpha.

So when was it actually clarified if Zilpha was Horace Delaney's or Salish's daughter, if it was? (I doubt we know anything about her other parent, though?)