Tiara is fine. I grew up with one so I think of a person before I think of the crown tbh. Maybe the mother liked the sound over the royal quality but with a princess for a sister that may not be true.
Anya as a nn for
Anastasia is great. I love it.
Anastasia by itself is okay. I used to love it but now the "stas" part sounds off to me.
Anya is much more fun. Biased, though, as
Buffy the Vampire Slayer is my favorite show and the character
Anya is a hoot and a half on it.