Exactly how steamy is this story?
PG-13
There is some sexual language and behavior in A Northern Light, but it's not super explicit. Sex in the novel is seriously located in the early twentieth century, with all the twentieth century norms and social standards.
So when Royal puts some moves on Mattie (ew), she has no idea what to do. And when Mattie asks her mother where her virtue is (37.7) and gets a vague, blushing answer, we realize that for as gritty and real that a rural community might be with its sexual drama between Frank Loomis and Emmie Hubbard and its animal husbandry practices, talk about sex between humans is still very much socially verboten, even among family members. Plus Mattie's too innocent and interested in other things to be preoccupied by sex.
On the other hand, the consequences of sex overshadow much of the book: Frank Loomis's sexual relationship with Emmie Hubbard affects his children's behavior; Mattie is swayed by her budding sexuality with Royal; and Minnie's sexual congress with her husband produces twins, a horrific birth, and some serious post-partum depression which deeply affects how Mattie views marriage. And then, of course, we've got Grace Brown getting knocked up out of wedlock by Chester Gillette, and murdered as a result.
So while the sexual nature of humans on any given page is pretty tamely described, with a few exceptions, sex does permeate the novel. Hence the PG-13 rating.