Aurora does not know about Nasuka, but Nasuka knows about Aurora!
In more detail, Malva actually isn’t aware of Nasuka, or at least has never confirmed that Nasuka is her daughter. She hooked up with Ryuki during one of his tours in Kalos, but he’d moved on by the time he even realized he was pregnant, so Nasuka is either born on the road or back in Alola, with Malva none the wiser. As such, Aurora doesn’t know about her half-sister since Malva, not knowing herself, would have never mentioned it.
Nasuka on the other hand, while not 100% sure, has a pretty good idea that Malva is her mother. Ryuki never confirmed it was her, just saying that Nasuka’s mother was someone he met on tour and had a brief fling with, but based on Nasuka’s own looks and what she can piece together from her father’s account, she’s able to figure out that Malva might be the one. So, when she looked up what Malva was doing currently, Nasuka found out she had married Glacia and had a daughter, who would most likely be her half-sister. She’s not sure yet what to do with the information.