The Second Summoning by Tanya Huff is the second book in her The Keeper's Chronicles series. If you haven't read the first book, Summon the Keeper, you'll be lost. You really need to have read it to get many of the things referenced in this book.
Claire Hansen is a Keeper, a person with magical powers charged with preventing hell from manifesting on Earth, and she takes her job seriously. Too seriously. She's kind of a stick in the mud. She finally loosens up to the point where she consummates her relationship with her non-magical boyfriend, Dean. I mention it because the Earth moves. And heaven weeps. Sort of.
While Dean and Claire are occupied a teen aged girl brings forth a manifestation of her version of an angel. She's been reading Twilight and watching Vampire Diaries. The angel appears as a teen aged boy who is, as they say in Star Trek, fully functional. And because of the "as above, so below" rule a demon is brought forth in the form of a teen aged girl.
While Claire goes after the demon, her teen aged sister, Diana, goes after the angel. Claire intends to send both of them back where they belong. She does not consider them people but as manifestations of good and evil. The problem is that things are not that simple.
Tanya Huff is one of my favorite urban fantasy writers. Even though I find Claire and Diana annoying in many ways, I still enjoy the Keeper books. I will say that I don't think this book is as good as the first one. It seems to be a little light weight. Maybe I just don't have all that much interest in teenagers.