I think that the journals do tend to line up with the artifacts that Katie is finding. No, Katie doesn't get the specifics that the player is getting, but she is getting a lot from the artifacts. There is evidence of the romance, Lonnie going away, the ghost hunting, her parents' lack of acceptance of the homosexuality, and her sister's general problems fitting in. More than anything, the first note that Katie receives puts her in the mindset that something is wrong. I think the game is making an assumption about two things. The first and big assumption is that Katie is able to put the pieces together and understand the basic narrative of her sister's romance even if she doesn't have all the pieces. The second assumption is that the player is picking up at least enough of the artifacts that Katie could come to these conclusions. That might be problematic, but considering the nature of the game, it's not an unfair thing to assume.
Essentially, the audio ensures that the player is in-line with Katie in terms of thought process. Yes, we're getting more info than Katie is, but it's only to accomplish two big things. One is to make sure that we understand Katie's thought process and piecing together. The second thing is to really know and care about Katie's sister. Once again, Katie would be worried the second she saw that note. Our greater understanding of her sister through those audio logs is meant to let us have something close to the understanding Katie has of her sister. It makes us as worried and as concerned as our protagonist, not more-so.
For me, playing the game, everything rung very true. I actually did what Katie would do. I immediately went to the phone, couldn't use it. I then scattered around and turned on every single light in the house, seeing if anyone was there. Then I was left with this initial cryptic note, a missing family, and several locked doors. In my mindset, I was looking through things primarily because I wanted to open those doors which is the big progression of the game.
The only moment when I feel that the character is distanced from Katie is the ending as Katie won't feel catharsis until she reads the journal and comes to that last entry. Still, I feel like there was almost a purposeful distance established there. I knew Katie was going to read the diary, and I knew what she was going to read. It was almost like I was drifting away from the character, no longer inhabiting her for this moment in her life and letting her move on with that life.