For people who don't understand (spoilers)
The dad did have a fever. He was the one who made them sick. The spirit they kept seeing in the house/around the house was their mother, because she wanted to always be close by.
At the end, the Sheriff (who the dad tied up) said "When I first saw him he was so sick, half out of his mind with a fever, I suppose. He'll get what's coming to him" or something along those lines.
I don't understand what "he'll get what's coming to him" means exactly----but it was the dad who was sick, and gave it to his kids. He was sick the whole time and didn't know it. The Sheriff really was getting over whatever sickness was going around, but the dad didn't know.
He was hallucinating at the same time while taking care of the kids. Because of the meds/fever/drinking. The thing that bothers me with that---is I would buy the crazy hallucinating with that sort of combo if the mediation was NOT "Clonazepam." Makes sense he would be on it, but I take that for my severe anxiety disorder---and unless he was taking like 6 at a time of a very strong dose, with the alcohol, then yeah maybe he might hallucinate. But clonazepam isn't a drug that does that. It takes away anxiety. It's not nearly as strong as other drugs. If he was taking Seroquel or some kind of depression medication, bipolar medication or something like that--- then yeah, mixed with a really bad fever and alcohol, I could believe the hallucination.
The spirit of his wife was there in the house. When he said "I'm sorry I couldn't save you" you hear a whisper from a woman "But you did save them" and it went on to show exactly how everything really went down. So at the end with the curtain, and it looking like someone was watching them leave---it was his wife's spirit. They were seeing her there the whole time, but she looked so scary because he was hallucinating, and very upset when he thinks about her and the night she died. It was her eyes, through the shed, and her walking through the trees.
When he was hallucinating you also hear a man's voice saying "Please don't leave me here" during all his blurry madness. That was the sheriff calling from the attic begging to let him out.
It could have been a way better film had they changed the medication----and if the Sheriff didn't say "He will get whats coming to him" ---the Sheriff should have been a bit more understanding since he realized how sick he was at the gas station (they don't show that, to make it more of a twist) that he was very sick, and freaking out, and wasn't a bad person at all. So that line makes no sense, but other than that the rest makes sense. (If you believe clonazepam is a drug that will make you hallucinate like crazy with alcohol and a fever) Which I swear----it didn't show the dosage but it should have, it would have to be like a TON of it. Or he wouldn't be hallucinating, he would be dead. Like sleeping pills. Should have been a different medication.
Anyway, that is the movie in a nutshell even though it seems confusing. When you're in a sad/depressed state, your hallucinations are more likely to be scary, and in this case it was coming out as his worst fears----that he wasn't a good father and that he would somehow not save his kids from being sick. That was the first problem he noticed when they got there before his sickness got worse and he took the meds and drank.