I have seen a lot of corruption first hand, and my city actually had people do serious time for it, there is a documentary on it and there might be a movie. In short, I know what it looks like.
Most likely,the Sheriffs are corrupt, at least somewhat. You get this a lot with small sheriffs and cops. The problem is that a lot of smaller boroughs don't have strict hiring policies. That's the reason a lot of people you see on reddit who say "every cop I've ever met is nice" is usually from a big city, where the cops they see were heavily screened and have to compete for the position. That's why they're good.
With smaller boroughs, seeing favorites hired isn't uncommon. Usually it is a pretty straight shot. A guy/gal gets into a position of power. They hire a cop/sheriff who they like and know personally. Methodically, the rest of the staff is bad mouthed/trashed. Pretty soon, that guy who was the personal friend is now known as "the good one", and is line for promotions. This is how bad cops get hired in big cities. Lets say there is a guy who is fresh out of the academy but we know(because this is fictional) that he'll be Super Cop. He won't get hired because the small time well known guy is "experienced" and "was pretty close to becoming Chief", or in some cases did become chief.
Note: this isn't always what happens, but it happens enough that people notice it.
This is how it starts, this seems like what might have happened here. I looked up the towns involved and I noticed the county had 46,000 people. That isn't a huge city, let alone a decent sized county..
I believe 100% that the people involved in this altered evidence and are guilty of most of the things leveled against them. I do not believe they murdered the girl.
If I had to guess, and I do, I would assume the family of Candace probably wasn't well off. Single mother at 17(which means she was pregnant at 16, or earlier), no father in the picture for the family, and you started your story off with the brother being in trouble with the police. This isn't meant to be a patriarchal rant about the importance of fatherhood, it's meant to show that the family might not have the best reputation or judgement.
Seeing someone "collect envelopes" seems odd. Why would this girl have seen anything? This seems like something said that was just meant to antagonize a man she didn't like(perhaps with good reason). This wasn't exactly a reasonable response and after fighting with law enforcement she stated pretty much that she would accuse the officer of corruption. This is very noble... If the information is true. If not, then it is a crime.
When someone dies early, you run through your head what caused it. What could you do, what didn't you do. I'd imagine this is amplified time 10 with a murder, times 50 with a lost child, and times 10,000 with an unsolved murder of your child.
It is easy to make poor connections of evidence in your brain when someone in your family dies, and you cling to certain items that help prove your world view.
Combine those two, and you get someone who might be in denial. Especially if one child killed the other. It would be easy to reach for excuses, especially given the fact the police are an easy target.
But the murder doesn't make sense. So she was shot, then nearly beheaded, and hidden poorly? That doesn't seem like a very good idea if you were trying to get the girl to be silent and the whole thing to disappear.
A lot of this stuff seems to paint a very different picture than the one you drew yourself.
"The dog was tied to a tree"
Does that mean the dog was given some sort of execution? Tied like a noose and gallows, or tied like a dog would be to a tree to keep it from running away? Why kill the dog to send a message to a girl you planned to kill? And again, if this was a poor area, not to hammer it home or disparage the family, but dog fights are common. Plenty of dogs get kidnapped and held for this purpose, just look at the issues Los Angeles has with it, or even worse Fresno, CA.
Why did they need to do a manhunt for the brother. Doesn't that seem odd? The family appears to be close, I mean the sister said she would basically be arrested because the brother was being treated unfairly. All of a sudden the girl is brutally murdered and the brother disappears for 3 days?
Cover up? Sure. Murder? Doesn't seem very likely.
The answer is there, it is just very unpleasant to look at.