Is the issue of the vampire 'girl's' gender addressed? In the original she's not exactly a girl.

Also, does it have the 'happy ending' that the writer wanted to add on in a subsequent epilogue, as he didn't want it to be ambiguous as to whether the boy's fate would be the same as that of the old man, falling into a 'Renfield' type role, and rather that they were partners who were happy together, rather than him turning into an exploited servant?

It's actually ambiguous, but if anything it leads you to believe that Owen's fate will eventually be the same as the Father's. In an earlier scene, Owen is in Abby's apartment and he finds an old picture of Abby with what appears to be a child version of the Father. That said, the Father didn't seem like a "Renfield" or servant in this movie. He seemed to instead be someone who loved Abby with all his heart and was doing the terrible work needed to keep her fed because of that love. That said, years (decades?) of murdering innocent people for their blood tends to wear on the soul somewhat.

My view is that as long as it was done within a year or so (before puberty hits Chloe Moertz), they could easily do a sequel to "Let In" with the story moving to modern times with Abby moving on to another kid once a now 40-year old Owen begins to get tired, sloppy and soulsick of all the killing.

Okay. Thanks. In the original, the 'father' character is a pedophile that 'Eli' has 'recruited'. The author has said that he's unhappy that many people found the ending ambiguous and is writing an epilogue that clarifies that it's a 'happy ending'. This is going to be published imminently, but I'm not sure how long it'll be before a translated version comes out. It's supposed to be only ten pages long, but just clears up all of the unanswered questions definitively. From what the writer's said in interviews, it does seem that things will be different. I think the title of the epilogue story is 'Let the old ways die', which may be some indication that its' the case.

Have they skipped the gender issue completely in this version too, then? Is 'Abby' definitely a girl? Do you see any of hir history?

Like I said, the movie pretty much goes with the author's "happy ending" however, with the "Father" it shows the ultimate sadness of that ending. Like the Father, Owen will age and die, probably violently. The only way for Owen and Abby to stay together would be as relative equals. Either she'd be cured of vampirism, or he's become a vampire like her.

As for the gender question, there's no real ambiguity about it. Chloe Moertz dresses and acts like a girl. There's even a point where Abby gets into a bed naked with Owen (at which point he asks her to "go steady" with him). Since Owen did not suddenly ask her why she has a penis, I'm going with the idea that Abby's a girl. There is dialogue in the film where Abby denies being a girl at one point, but it seems clear that that is more a reflection of her awareness at being an unnatural creature that kills people for their blood and outlives those for whom she cares.

As for her history, there's nothing explicit said. Abby does own a pretty large collection of antique puzzles and games that look well-used. There's also a photo of her with the happy, smiling boy-version of the Father that seems to have been taken in the 1930s or 1940s (possibly 1950s as well, since Richard Jenkins was born in 1947). The implication seems to be that Abby has been a vampire for a long time, decades if not centuries.

All I can say is they didn't go that route in this movie. Really, you should just watch it for yourself. It's a good film that stands on its own merits even if it is a re-make.