Acting:
As the only 2 people in the entire film, the capability of the actors is very important. The brother was good. He was able to show the weight of the plot in a way that was adding to the story. It mattered to him. The sister was not there, I don't think she was really present, I could see her acting. And the tough persona was not palpable or natural to her and felt like she was aware she was in a film. It seemed like she was doing acting like a homework assignment and the director is what mattered to her, not the plot. The relationship between the brother and sister was oddly formal and respectful which made me wonder if the director knows this creates undesirable conditions and tension. The formality and niceness made see them as friends or neighbors or just not as family.. They should have been meaner and sloppier with each other, more down to earth and ugly, not just saying all the right things. The inherent imperfection of a sibling relationship wasn't there. There was no sense of self or history within this relationship, they only existed for the scenes. I realize it's a plot story not a character story, but this relationship carries all the stakes of the entire film. The film The Savages would have been a good place to begin research into developing the proper sibling dynamic and tone since it's the only 2 people carrying the entire film.
Writing & Direction
The script seemed like a first or second draft. There's just no meat on the bones. It's a poor man's Black Mirror episode. High concept with all kinds of lofty theoretical ideas but with none of the math to back it up. There's no heart aka no emotional investment other than the melodrama of the bad guys coming and a criminally underdeveloped idea of a multi verse. The writer needs to READ some of the great sci fi texts and books in history, I'm sure he's watched all the iconic films, but that's what happens when you give film buffs money-they are just trying to be like the other films rather than authoring their own world and rules and senses. Cinematically, a unique and interesting way to have approached this text would have been in the style of Leave The World Behind where the DP and director matched the shots and sound design with the oblique story. Or The OA, where the plot is multiverse but still congruent and tight, and not hiding its lazy underdeveloped hand by blaming interpretation. Sure there's validity in open interpretation endings, but this was inverse, everything was open ended the was no bible to this world. There is little reverence given to the plot itself and seems like they just threw spaghetti and the wall and saw what stuck and kept it. There's no rhyme or reason and at the end of the day, nothing to say. No vision. I don't need it to be allegorical or have metaphysical philosophical significance, but these are things you think about before writing a multiverse time travel movie. And if you don't, then you end up empty convolutions without a book of god and also, what a missed opportunity.