Maybe it’s bad form to slap a movie you like with the one thing you didn’t like about it, right from the top, but here goes.

There’s a third act turn toward the dark in “No Alternative” that feels abrupt, harsh and unearned — or at least less earned than writer-director William Dickerson may have felt.

It doesn’t spoil the coming-of-age period piece that precedes that turn, and trails it. But it does tend to blunt its impact.

“No Alternative” is about siblings growing up in affluent suburban New York in the post-Nirvana grunge era. It’s 1994, and Bridget (newcomer Michaela Cavazos) and Thomas (Conor Proft) aren’t just sister and brother. They’re tight.

He’s waiting to hear if he gets into Georgetown, and he’s protective of his kid sister. Bridget seems to need it. She’s moody, stuck in a stocking cap, day and night, and in mild-mannered foul-mouthed rebellion with their parents.

Dad (Harry Hamlin) is a semi-stern judge running for the state supreme court, the type to bring home graphic descriptions of the crimes and criminals showing up in his courtroom, and Mom (Kathryn Erbe) wants to dote, but gives her kids space.

As we’ve seen Bridget in therapy, we get it. Her doctor is all about “smoothing out” this by prescribing that, treating her as a chemistry experiment.

Tom’s killing time this winter of college acceptance letters by jamming with his punk band. When they meet raspy, brooding and handsome singer Elias (Aria Shahghasemi), they realize that maybe they can take this seriously.

“I have a song that’s four chords…”

“That’s one too many, if you asked me!”

“It’s a love song. Called ‘Chumming.’

Bridget rolls her eyes at grunge. She’s into hip hop. And a discarded portable keyboard inspires her to put down the cigarette, knocks back a Zoloft with a vodka chaser, and starts spitting some rhymes of her own.

Dickerson’s film follows their parallel paths, a sister and brother seeking different things from music. Tom might get to put off growing up just a tad longer. Bridget? It’s her release, an artistic outlet (she also paints) that captures her ironic rage.

“No Alternative” isn’t a comedy, exactly. But Bridget’s scenes, as Bri-Dab, a rapper claiming a fake Harlem background, taking on the persona of Free 2B, rapping in the voice of a young black man (including the N-word), are hilarious.

“That was so punk rock I can’t even handle it,” Sarah Lawrence College boy Stewart (Logan Georges) tells her. He’s studying surrealism as a concept and sobriety as a lifestyle. He’s sweet on Free 2B.

And this happens as Bridget’s classmate, the promiscuous and equally “punk rock” (in that “Sheena is a Punk Rocker” sense) Jackie (Chloë Levine) is coming-on-strong to young Tom.

Pressure starts to build as the band bickers and the true extent of Bridget’s illness becomes clear and distracted Dad gets death threats, something he levels with his kids about even as he’s taking the strain out on them.

Maybe “You have my legs. You have my father’s legs. They’ve carried us a long way,” isn’t the platitude his son or daughter need at this moment.

Dickerson (“Don’t Look Back” and “The Mirror” were two earlier indie efforts) is most at home immersing us in the milieu, and sending it up.

The “band” is more an excuse to hang out and drink and smoke and get tight and debate the qualities of this new film (“Pulp Fiction”) and where the filmmaker “stole” his ideas from.

Everybody’s worked up about how “punk rock” this or that is, and Tom and his pals are dressing and playing as if they could be “the New Nirvana.”

Bridget is adrift, impulsive, medicating and self-medicating. But she’s more in sync with the winds of change in music. Maybe she and her brother are drifting apart, musically, but if Jackie sleeps with him and smirks “I’ve had better,” she’s asking for a beat down.

Cavazos, who has the swagger and gift for the rude and crude of a young Sarah Silverman, is a revelation. Proft’s character is more a “type,” but he makes do with that.

There was a longer cut of “No Alternative,” according to IMDb. Perhaps the lost footage smoothed out and wholly-motivated that abrupt and clumsy third-act jump I opened the review complaining about. Perhaps not.

But what’s here is still a promising, entertaining effort. And it’s a fine showcase for Cavazos, who nails Bridget’s vocal fry, her pose, the disturbed and self-destructive vibe that she wears like a stocking cap, her armor against a world her illness — meds or no meds — won’t let her master.

MPAA Rating: unrated, sex, teen drinking, drugs

Cast: Michaela Cavazos, Conor Proft, Kathryn Erbe and Harry Hamlin

Credits: Written and directed by William Dickerson. A Gravitas release.

Running time: 1:37