Making a movie like this work is a delicate balancing act, and it almost worked, aside from the ending. The movie is really about an upper middle-class family - much like American Beauty - comprised of: a toxic mother, a codependent father, and the two sisters - Grace, and Pauline. Pauline is our lead character, and her story plays out like a coming-of-age indie comedy - like Welcome to the Dollhouse; she's smart, socially awkward, interested in biology, and has no friends aside from her little sister.
The major issue with Excision is that there is no tonal center: is it a drama? no, it's too weird. is it a comedy? no, it's too dark. is it an indie dark comedy? sort of, until it does off the rails in the third act, and dives headfirst into horror.
[spoiler]Okay, consider that aside from the jammed-in fantasy/dream sequences, Pauline is very smart, and cares deeply for her sister. How much better would the ending have been if Grace was shown to have survived the surgery? Pauline would still have been a murderer - of the unwilling donor - yet it leaves us with some sympathy for her, knowing that she was completely misunderstood by everyone, but would do anything to help her sister, despite the consequences.[/spoiler]