I think Deb is going to become more of the protagonist, as the show has kind of been heading in that direction for a while. At the very least, we'll continue having each episode split pretty evenly between the two. I think she's going to start putting things together. Remember that there is still an entire Mitchell Family alive to point at Dexter and say that he was around for months, and that he was in the house when the police broke in. That would be enough for her to start searching.
Then maybe she starts digging around. The big moment is going to have to be when she realizes that Doakes wasn't the Bay Harbor Butcher. Maybe all it takes is for his family to point out that he has alibis for MANY of the murder victims. Then she puts the pieces together and everything starts pointing at her brother.
I think she'll have to turn him in... Deb has many faults, but she always does the right thing in the end, no matter how much it ends up hurting her. Just imagine the moment she's searching for evidence in his back shed and finds the murder instruments. Or when she realizes what exactly he used the giant walk-in freezer for in his old apartment. She sees that she has the same old model of air conditioner, and maybe its noted for how easily the front falls out, and she sees that rather than filled with dust, this little open area is impeccably clean. She sees the same one in her brother's shed... There are a LOT of ways she could figure it out, but I can't see her just accepting it. She would confront him first before bringing in the evidence, but she would DEFINITELY bring in the evidence. Either he'll run or be caught. Before any of that he'll tell her about the code and what her father did and how he shaped him.
I think the best way the show could end would be Deb having to raise the kids. Maybe a final episode cuts forward several years and we see a young harrison having just killed the family pet, looking at the blood on his hands. And there's this fascination in his eyes.
Then Debra needs to make a choice. She knows about the code. If this kid winds up turning into a monster he'll be caught immediately, but since she loves him, she wonders what might happen if she pushes him into becoming the right sort of person.
THAT is how I think the show will end.
