As a society, we do a terrible job of talking about mental illness. We do a worse job caring for people struggling to recover from their illness or to manage their symptoms. In Sandra Luckow’s personal documentary, “That Way Madness Lies,” viewers get to see just how ill-equipped healthcare and law enforcement officials are at helping some of their most vulnerable populations.

Sandra and her brother Duanne grew up in the Pacific Northwest in the ’80s. The documentary dotes on these fond memories and photos of family ski trips near Mount St. Helens, and how Duanne’s love of film helped inspire Sandra to pick up a camera. The four members of the family were all creative in their own way: their father, Gerald, liked to restore cars, which Duanne later made his business. Their mom, Dolores, spent her free time making miniature homes and models. Before Sandra dove into the world of filmmaking, she went through a ventriloquist phase.

Over the course of the film, Sandra asks various questions in her voiceover narration as her brother’s illness progresses. What was the “line between creativity and crazy?” Is he still harmless? How much more can she take of her brother’s harassing emails, his involuntary commitment, astronomical hospital bills and hidden secrets?

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Not all of Sandra’s questions are answered, and her uncertainty in these moments is palpable. They’re the musings of a person looking to help their loved one and not knowing how to do so. Her voice carries hints of frustration, fear and so much pain. As Sandra’s film damningly shows, there’s no playbook for how to deal with a loved one’s severe mental illness in our complicated healthcare system, and the law only really shows up when things get dangerous.

Part of Sandra’s problem is that Duanne defies classification, alternating between periods of peace and dramatic chaos with little warning — or perhaps warning signs that every one of his loved ones missed. There’s a friend who tries to help him, who ends up finding the police at his door. Duanne’s parents are in shock when they learn he’s been conned out of thousands by a shady email spammer in Nigeria. It’s just as painful to watch Duanne descend from a willing participant in his sister’s documentary to threatening her and upending the family’s lives.

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As he retreats further into conspiracy theories and obsessions with women who don’t know him, Duanne’s voice disappears from the documentary, becoming a stranger in his own movie. It’s a sense of what Sandra loses in her life, and his lack of participation somewhat hobbles the second half of the “Madness.” He becomes a mystery both to those around him and to the audience.

While Sandra’s voice-over narration quickly guides the viewer through years of a complicated story and cyclical trips to mental state hospitals, it can feel like the film is leading the audience too forcefully. It’s possible to ask too many questions or to offer too many details that lead viewers away from the main subject, Duanne.

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The documentary’s use of family photos, Duanne’s DIY movies, video diaries and Sandra’s present-day interviews reveals a bigger picture than just the lives of the Luckow family. “That Way Madness Lies” is very much about the isolation, insecurity and regret family members feel when their loved one takes a turn for the worst. It’s also very interested in looking at how terribly easy it is for people to fall through the cracks every day.

Sandra also interviews a number of Duanne’s friends to get a fuller picture of her now-reluctant subject. While the siblings’ father remains mostly tight-lipped about his son’s illness, Sandra’s conversations with her mom become something of a turning point in the film. She’s forced to stand up to her son’s demands, even as she admits she never saw any reason to get him help when he was younger or when he was in his 30s and still living with his parents.

Near the start of the film, Sandra proposes this film project to be a way for her and her estranged brother to bond. Those early scenes end up being some of the happiest in the film, something we didn’t know to appreciate before it disappeared. Aside from a few technical choices, like heavy narration or so many travel shots out a plane window, the documentary’s narrative is a compelling story that’s full of so many emotions. Its uncertainty makes it a gripping watch, yet the film’s introspection also has a profound effect that doesn’t turn it into a message movie.

Over the course of the film, the focus evolves from just following Duanne and his illness to the journey of two siblings in this new stage of their lives. In its modest efforts, “That Way Madness Lies” embraces a kind of sensitive nuance you don’t always see in depictions of mental illness in the movies.