Did I miss something?

"the brother reveal was too early and diffused most of the tension. then the neighbor storyline that went nowhere, followed by an ending that made no sense and felt disconnected from the rest of the film."

Actually I feel as though the brother reveal could have been done even earlier on instead of drawn out later in the movie. That would have left room for more unexplained phenomena inside the house and maybe elevated the creepy factor. I also feel the whole neighbor thing was a waste of time and served no purpose whatsoever in the story.

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Events in the film do tie together, but not in a way that's explicit, much of it is suggestion.
The scene at Lake Mungo, for example, I saw as Alice seeing the physical manifestation of her double life / the secret part she didn't share with her family (the affair with the neighbour etc.) This is foreshadowed right at the beginning of the film, as the titles appeared, with the old 'ghost' photographs, and the dialogue that would appear later in the film "I feel like something bad is going to happen to me", "Alice kept secrets a secret".

Similarly the juxtaposition scene where Alice and her mother's consultations with Ray, 'the psychic', (whose psychic abilities I think are probably false), where Alice and her Mother are describing as having exactly the same dream, albeit at 2 separate times, from their own perspective. This put me in mind of the earlier observation in the film, from the characters, that Alice was like June, and, in turn June was like her mother, that there was a privacy that they couldn't share - but that there was an understanding, or perhaps a tension between them.

You're right about the mood of the film being dreary and morose, especially given the subject matter, it's fitting that it is.

I think it would've been the easy option for the film makers to insert some talking heads, or 'experts' at the end of the film, to neatly explain to the audience what they had seen. But it makes the film much more intriguing that the audience is left to piece the jigsaw together.

It is a slow moving film, told from different perspectives, at different times. That's okay with me, the story arc, the fantastic acting, and intrigue carried me through this. We never got introduced to Alice, we only knew her after she died, so the audience is reliant on other's interpretations. I did think the whole brother faking the evidence was too late in the film, it would've been better had it happened sooner; perhaps the addition of other secrets Alice was keeping might've added weight to her sense of what was coming; and the quality of some of the shots (e.g. mobile phone footage) was unclear - but those are minor quibbles.

I did like the fact that as the end credits rolled, the ghost of Alice was revealed at various points (e.g. her brother's birthday party). The family didn't see her, reinforcing the sense given during the film, that Alice felt invisible and alienated throughout her life.

That's my interpretation.

Yup. Me too. I feel like I didn't get the whole scene at lake mungo.

It kept my interest throughout but I didn't find it scary or creepy. I did think the acting was very good and it was well made; the first ten minutes I had to stop it and look up whether i was actually watching a documentary.