The hints towards her being... alternately-genitaled.
Do you think it's something that just happens as part of turning into a vampire? Or was she born hermaphroditic as a human and stayed that way? Or maybe hinting she's trans? Though I think that less likely given her supposed age and the amount of understanding there was towards trans people then.
I don't think that vampires have a true gender. They are, after all, undead, and, according to legend, the flesh is an illusion. They can look like anything that they want to look like.
So, Abby chose to look like what she probably was when she was turned as a child. After all, it was a good disguise, looking like a helpless twelve year old girl.
She’s definitely female. When she says “what if I wasn’t a girl”, she’s trying to hint that she’s not human.
shareIn the original and the book, Abby was instead a boy named Eli who was castrated after being turned into a vampire and was androgenous enough to pass for a girl. He never said he was a girl, people just assumed he was. There is a similar scene where Oskar (Owen) spies on him while he's dressing and sees his castration scar. It's made very clear.
In this version, Abby seems to be a girl. She does tell him she's not a girl which could mean she is a boy or it could mean because she is a vampire. In the scene where Owen watches her get dressed, he seems to look a bit surprised. This could be for the same reason as the orginal or it could be because he is seeing her naked.