I would be thrilled to find out just about anyone other than RH is that girl's father. The description for Robyn said she is trying to escape the shadow of her parents, so it seems like she has been with Zelena her entire life. I think it's unlikely she will turn out to be wish Hook's daughter, but you would certainly not hear any complaints from me if that did happen. I wouldn't care if there were a thousand plot holes involved, I'd still accept it.
It does seem like Zelena and Robyn will tie into the story with the witch (or Eloise, if she is not the witch) somehow.