Appropriate to its title, there is a rather steamy moment of pre-sexual activity, but the narrative breaks away before anything actually happens. I doubt that it's anything that a 12 (or 13) year old hasn't seen in a dozen PG rated movies or read in a Harlequin-type romance novel, so I think she'll enjoy it.
If you have any doubts, I'd recommend reading the ending of Chapter 9/beginning of Chapter 10 first (available free on abc.com: http://abc.go.com/shows/castle/castle-novel). Then you'll be able to decide if it is something appropriate for your daughters to read. There are a few bad words here and there but nothing too crazy.
I would have read it at 13 and not had a problem but didn't got any sophisticated humor, nuance, etc., until I was more of an adult. Okay, I was slow in some areas, but anyway, it's hard to say. There was some sex in it (don't want to say more and spoil it) but there's lots more on some prime time televisions series of late. What always me as a sensitive teenager was violence rather than sex in books and movies.