In the interview, Parrilla says that as we move forward with this story line, other things will be revealed, and even though Regina is still disbelieving that she has a sister, it will turn out to be factual, and there won't be any denying it. Regina will feel betrayed, yet again, by her mother and it will be quite a shock that she never told her such a painful secret.
It was also suggested that there will be another significant plot twist involving The Wicked Witch of the West and Regina. Many fans have suggested that it has something to do with Regina being The Wicked Witch of the East, as Grumpy says, the kind you drop a house on. Parrilla says that it's not out of the realm of possibilities and you shouldn't rule it out.
There could be other major plot twists, such as the father of Zelena but at this point fans are just going to have to wait and see. There is going to be a lot of tension between these two sisters and it's unlikely that they will ever like each other, especially since Regina has told Robin Hood that her new purpose in life is to take out her sister. But could Regina really do that now that she knows that Zelena is her sister? Or would it be for the greater good that it be done?