Why does everyone think Luna is Stella?

[Fynn]

So after my time with the demo I'd say it's safe to say that Luna IS Stella. It was simply a name change.

Unless they were both at the same place when the city was attacked, they both barely made it out of there, both of there middle names are Nox and (SPOILER)only one of them was mentioned on the radio after the attack for some reason (which would be odd if they were sisters)

I don't think anything we say in the demo contradicts the 'They are sisters' hypothesis. (SPOILER)Sisters will share names. They're from Tenebrae, not Lucis, so it makes perfect sense that only one of them was there, the other is still safe at home.