Sam Wescott Yeah, I might veer away from this one until she is comfortable reading sex scenes. I would recommend Gail Carriger's other series - Finishing School. It's a YA series and so far there has been nothing more than a palm kiss and minimal violence. Same absurd language, same world, but, like, 30 years earlier. Same supernatural stuff and similarly wonderful female protagonist. I love it and would not hesitate to recommend it to a 12 year old.
Bethany I'd say absolutely not. I'm 21, and it was far too graphic for me as far as sexual content goes, not to mention the violence. A book can be great without the graphic stuff. If she really wants something similar, a series that has some of the same characters and is set in the same world is the Finishing School series, although that gets a bit odd, too. (Note: There is some offhand discussion of private parts in the later books of that series as well.)
Sigour Definitely not. This is not a child reading... mostly because the book is not going to be understood properly. If you don't let a 12 yo watch a movie revolving around sex, female oppression, and abusive mothers, you shouldn't let a 12 yo read books like this. BUT, I am a very careful young mother, and I don't expose my children carelessly to things. Better start reading the Neverending story, Harry Potter or anything I have read beforehand and I am sure is not going to spoil the broth.
Tigress I read this when I was 13, and loved it. There is a scene where Maccon has just changed from wolf to man and thus is naked, and there's some innuendo there, as well as a sex scene at the very end (after they've been married, if that's of any importance to you). However, they're written in such a way that at that age, with my limited understanding of those matters, I didn't even realize that they'd had sex. And it's definitely not a racy or graphic series--any of the sexual scenes are written in more of a humorous way, and are fairly vague. Personally, I think the Finishing School series is a bit more frank with the discussions about sex and body parts, and isn't anywhere near as fun as this one. However, that series is also great, and has a strong 14-year-old heroine, so maybe you'd be more comfortable starting there.
Marshall Clowers If she read "Breaking Dawn" or the "Divergent" series, she might find this pretty tame. I'd rate it as a pg-13 series as there is more innuendo than graphic descriptions.
K Idamari It contains a sprinkling of very intimate scenes which are although described with an Wodehousian humour, are still of explicit nature. (It's about a 26 y.o. spinster who dabbles in the world of the undead in Victorian London.)
Nina It was not graphic at all, but highly suggestive - stick to her YA series (Etiquette and Espionage). She might also enjoy 'A clockwork scarab' and the Jackaby series.