I have had a theory recently that people have been talking about, but I think it's true. I believe that Opaline is Sunny's mother, but I also believe that Misty is her sister.
Misty looks like a mix between Argyle and Opaline, I truly think that they're the parents of Sunny and Misty.

Alternatively, Misty could be the daughter of Opaline and Argyle while Sunny was adopted. Maybe Argyle left Opaline with Misty, but she somehow took her back. Argyle found Sunny and started to teach her about the past so she could help the world.

If this is too long here's a summary; Opaline and Argyle are the parents of Misty and possibly the parents of Sunny.