All these discussions about incest are just western hysteria. Hatsune doesn't act like Saki's relative. Even when Saki talked about her mother, she's technically Hatsune's sister, Hatsune was more worried about Saki's condition than about her already dead sister
Because she doesn't see her as sister (she didn't even know her), doesn't see herself as aunt, and of course doesn't see Saki as own niece. She's the same age as Saki herself, she just a kid