Linda Lavin makes her last curtain call as graceful as the material will allow in “One Big Happy Family,” her final film before dying last December.

It’s a generally mirthless comedy that strains for laughs as it struggles to graft a “Maybe I’m not Jewish” DNA test narrative onto a child’s impending bat mitzvah.

Lisa Brenner wrote and stars in “Family” as an actress and mom who’s made being Jewish a central tenet of her life, only to discover that decades of “You don’t LOOK Jewish” remarks are now backed up by science.

Her story is told in a kvetching, klutzy script that botches basic math, struggles to make the whole DNA storyline funny, fumbles the bat mitzvah and saddles the 87 year-old Lavin with a veritable dictionary of Yiddishisms to utter.

“Oy” and “gevalt” indeed.

In a voice-over prologue, Rachel remembers freezing up during her own bat mitzvah, only to have her tactless, tone deaf stereotype of a mother (Lavin) take over her speech and scar her for life. The story then shifts to the present day, “twenty seven years later,” as Rachel struggles with organizing the celebration and with the idea of making a speech at her own daughter’s (Lumi Pollack) bat mitzvah.

She’s supposed to be an actress?

The DNA test thing is pushed on her by her TV chat show hostess bestie (Sabrina Cofield), and that’s when she learns she has a half-brother (Josh Fadem), or at least one half-brother for starters.

That infuriates her Calif-flaky, non-binary, juice-bar server/lesbian folk rocker sister (Kat Cunning, funny). But her Long Island mom isn’t admitting anything. At first.

“Did your sister give you one of her brownies?”

Mom then confesses to difficulties getting pregnant back in “the late ’70s,” until turning to artificial insemination.

That’s where the movie’s math, contorted to put a vibrant, doesn’t-look-it 87 year old in the mother/grandmother role. If Rachel is 40, she was born in ’85. Lavin’s Lenore is overusing Yiddish and talking about “remembering when the Dodgers were in Brooklyn,” when they moved to LA in ’57.

Possible? OK. But “plausible” demands more explanation.

The whole mass insemination story that produces constant DNA test site phone updates (amusing) revealing more and more siblings points to more interesting possibilities than it delivers.

Rachel curses in front of her kids and they curse back. Rachel’s sympathetic Filipino doctor-husband (Dante Brasco) keeps suggesting LA scent and herbal remedies to her stress.

Casting Fadem as her non-Jewish half-brother Bobby just muddies the movie’s ethnocentrism. Bobby’s the most “Jewish looking” character in it.

But sitcom-veteran Lavin navigates the abrasive tactlessness of the archetype she’s playing with ease, even if the Yiddishisms feel forced and dated a generation older than the character she’s supposed to be playing.

Rating: unrated, profanity

Cast: Lisa Brenner, Dante Basco, Lumi Pollack, Josh Fadem, Kat Cunning and Linda Lavin

Credits: Directed by Matt Sohn, scripted by Lisa Brenner. An Electric Films release.

Running time: 1:22