What this movie is saying is this
Sara's sadness of drifting away from her twin (the event that divided them) was greater than the sadness Jess carried of witnessing their parents death.
I thought this was a really interesting theme and kind of unique. And no this movie clearly is not about split personality, because that would go completely against what it was trying to say.
Also the "split personality" theory is debunked by the guide being haunted by Sara at the end. (Or the sadness of it, meaning he is no longer safe in the forest)
- Editor of many movies you've probably noticed on Netflix but never actually watched.