I interpreted this film in a way I haven’t seen others do so far. To me this film is structured comparable to Mulholland Dr. That film’s events are a trip inside Betty/Naomi Watts’ broken schizophrenic mind. I believe this film is intended to be Agatha’s in a similar way.

Under this interpretation, my theory is this. Agatha’s mother, father and brother died the night she set the fire. She says in the film she drugged them before setting the fire. It’s never explained how they got out, because they didn’t. If they survived for real, why would Cronenberg even make her say she drugged her family?

So if I'm right, what we get in this film her mental fantasy sometime in the future, perhaps still 7 years later. Presumably, she is still locked in a mental hospital the whole time.

Here are my reasons why I believe this:

Her parents: One of the biggest clues IMO is what happens to her parents, starting with her mother. Her mother self-combusts (?), incinerates to the black bone, doesn’t jump in the water when she’s on fire, etc. What’s going on? Why include such an unrealistic looking, CGI heavy death? Because this isn’t a real death, it’s Agatha burning her mother to death in her mind. Likewise Benjie finds the father motionless on the chair. I see this as representing her father’s death in the fire either from the pills or smoke. Agatha wanted her mother to have a more painful and brutal death because her mother, not Havana’s, was the sexual abuser. It’s just in this fantasy the sexual abuse representation got told through Havana instead. We know that Havana's mother and Agatha's mothers were already partly interchangeable by both dying in a fire.

Benjie/Jerome: One of the keys to this Agatha story is she’s clearly in love with her brother Benjie. I believe part of her hate for her parents and part of what makes her eventually snap, may be deceiving her into believing they knew about their relations when they got married, instead of it being a crazy accident. By telling Agatha it was always a choice, Agatha internalized that it’s ok to be in love with and marry Benji.

I see Jerome as in Agatha’s fantasy, as representing a grown up version of Benjie. In addition to obvious appearance similarities, they are both actors, and both are romantic fixations of Agatha. Remember that Agatha keeps reading a poem implying there's only ONE love for her, forever and ever. So if her true love is Benjie, why is she off sleeping with a random driver? Unless it's supposed to represent Benjie. Except in this fantasy, Agatha gets what she wants, which is for Jerome/Benjie to like her as much back and sleep with her - do everything she ever wanted physically to her. More-so, the young version of Benjie in this fantasy, suspiciously has no interest in any other female skanks his own age. I suspect the opposite of what was the literal truth: The famous 13 year old actor Benji crawling in whatever groupie tail he can get and having no interest romantically in his deranged 18 year old sister. Also, in Agatha’s fantasy Benjie is more like her by being a murderous crazy person about to be sent away too, instead of an actor with the world in front of him.

If Benjie=Jerome, what makes Agatha snap and set the fire makes sense because of the scene we see of Jerome sleeping with Havana. Say Agatha catches Benjie sleeping with an actress, an older patient of her father’s at that. Now her dream of Benjie being in love with her and marrying her is shattered. In addition that it was an older actress abusing an underaged Benjie, it taps into the sexual abuse past she had with her parents. THIS is what makes her snap, first beating the actress to death, and then going all the way with the fire that kills her mother, father and brother. The brother not burned to death, but by pills after a “marriage”, most likely with him unconscience or already dead instead of rewritten as consciencely like in her fantasy.

The hardest part to put together is how does Havana tie into Agatha’s fantasy? How much of Havana in this fantasy was literally real? Did Agatha really get an assistant job with an actress/father’s patient, then kill that actress after she slept with Benjie? Or was Havana almost entirely created by Agatha? There’s connections between them like the poem Agatha is so obsessed with, or how Havana’s mother died in a fire. This makes me think Havana is partly meant to represent Agatha the most of these characters. Figuring out how the ghosts visiting Havana and Benjie tie in is tricky. I feel these ghosts are speaking not to these characters but to Agatha. It’s the closest Agatha comes to snapping out of her fantasy and realizing this stuff isn’t real anymore. That’s why these ghosts scare the crap out of her.

Before laughing this off, considering all the facts showing how this is NOT a straight and forward literal tale. The ghosts, the mother’s incineration, the connections like the poem and Havana’s mother death in a fire, the Jerome-Benjie similarities, etc. I feel there’s something clearly afoot here, whether my interpretation is right or not.

edit - Just wanted to include another clue I read pointed out another board, how come Benjie gets visited by the ghost of Mika/the drowned kid? Not only is he irrelevant to Benjie, but in the scene Havana finds out, it's stated that nobody else knows he's dead yet. But it makes sense if this is all in Agatha's mind anyways.