In previous seasons, Deb's waded around her brother's bloodbath many a time and not even realized it. How is she doing now that she's actively covering up Dexter's misdeeds? Harry may be right in that sooner or later it's going to be too much for her, but she sure is handling things well so far.
"We are in a storm of fuck."
With Deb, Dexter has two options. The first is to try to keep as much of the truth as he can from Deb in the hope that she'll turn a blind eye to what she knows about him. But doing so may put her in danger. The second option is to tell her the things she needs to know to keep her safe - like that he found and didn't report Viktor's finger print on the car left at the Mike Anderson scene, that he killed Viktor and that Isaak knows he killed Viktor. And that now Isaak may be targeting her and anyone else he thinks was working with Dexter to kill Viktor. See, that last part is the most crucial bit of information, but he couldn't just tell her that and not end up telling her everything leading up to it. Of course Deb is going to ask questions. After all, she's a cop.
We'll get to talking about Viktor in a little bit, but for now, the point is that Dexter is giving Deb a flood of answers - telling her things she doesn't want to know and contradicting past explanations in an effort to keep her safe. Going after Speltzer was one thing. He slipped through the legal cracks, but Dexter intentionally kept Miami Metro from tracking down Viktor so that he could kill him. There's a difference and Deb knows it.
Dexter's admission to all this had Deb reeling as she counted up the murders she's helped cover up since finding Dexter with Travis. The bodies certainly are stacking up and things have only gotten more complicated. Factor in Deb learning that LaGuerta's conducting a private investigation based on a blood slide she found at Travis' murder scene and things only get worse. When Masuka brought Deb in on this, fearing his job was in jeopardy, Deb confronted LaGuerta and immediately climbed on board the investigation, using the fact that she was Doakes' partner as excuse to want to help LaGuerta, and claiming she believed Doakes was a good man.
LaGuerta had a list of missing persons whose information lined up with the Bay Harbor Butcher victim profile. Deb took the list to Dexter and he pointed out three men he killed, one of whom was a wedding photographer named Barnes. Deb immediately worried that Dexter might have been caught on camera while stalking Barnes at the wedding and sure enough, he was.
As though to confirm that Barnes was a list-worthy bad guy, a trip to Barnes' family's house and an encounter with his son revealed that not only did the guy like to secretly kill out-of-towner wedding guests but he also beat his family. At the very least, that eliminated the sympathy factor for Deb. She didn't have to sit there staring at a kid who lost his father and feel guilty. That kid may as well have sang a song about his father's absence.
The point of the visit was the wedding photos and it only took Deb a few seconds to spot her brother in one of them. There was Dexter standing about an inch away from the bride (really stealthy, Dexter. Did you help cut the cake too?) and later showed it to Dexter to point out that she covered for him, and maybe for a little I told you so.
All of the emphasis on Deb and Dexter reminiscing about their fun-in-the-sun days growing up seems to suggest that Deb's thinking about the bond they share and the history they have. Maybe those conversations are just a reminder to us of why she's going out of her way to help him now because other than the comments about the way he looked out for her when they were growing up, she seems pretty unraveled by her participation in covering up Dexter's crimes. And we can also add awkward quasi-sexual tension on Deb's end as she glanced awkwardly at a shirtless Dexter while he climbed into the motel bed next to hers.