Rita is only mentioned a handful of times in the book. There is one part where Dexter sees Rita every night, to make Doakes get bored of following him, and you can kind of see that Dexter is using Rita as part of his "normal" disguise. In the series, Rita is more of a main character. I've just started the second book, so I'm sure she plays a bigger role in it...
I won't kill my sister...I won't kill my sister...I won't kill my sister...