This story creates a very confusing atmosphere but don't let this push you away from seeing the film. This sense of "you don't know who is who" is exactly what you and the main character are feeling. The people who claim this film just "doesn't make sense", obviously don't see the struggles and meaning behind this film.
The director, Florian Zeller, and the actor, Anthony Hopkins, do an AMAZING job with the dementia feeling of everything. I don't watch much movies, and was just required for class but I just fell in love with it. Very strong actors, very strong emotions. Be prepared to feel anxiety, anger, confused and especially sadness.