Who is the "Mermaid Sister" ?

According on the new Lore of Ornn, we know that and are part of his family, BUT, it mention an another character who's called as "Mermaid sister" in the stories, and she is controlling rivers and sea, or something like that.

So i'm questioning, the only mermaid looking species we know are the kind of Nami, a vastayan undersea tribe called the Marai, are they related ? Are we gonna see her in the future ? I know it's not an important character in this story and she can just be another lore character like many others...

BUT, I think it could be so interesting to see this character in the future due to her link with Ornn, Volibear and Anivia, what do you think ? Can we have a Mermaid Goddess ? And I mean real mermaid, because Nami's species are vastayan, half-human and half Fish, even Karthus mention that his voice is more powerful than mermaid's sings, Is Nami and Ornn's mermaid sister are the same species ?