In the Mexican community, a girl’s quinceañera is like prom, sweet sixteen and a coming out party all rolled into one, with parents spending thousands of dollars to escort their 15-year-old princesses into womanhood. “Quinceañera,” an upbeat gem of a film that took both the grand jury prize and the audience award at Sundance this year, is bookended by two such celebrations. In between is a moving tribute to something else common in the Mexican community: a devotion to family.

This particular family, whose older generations were born in Mexico while the children and grandchildren are native to L.A.’s Echo Park neighborhood, is somewhat fractured at the outset. It’s Eileen’s big day, complete with bridesmaid-style entourage and tuxedoed male counterparts, but her brother Carlos (Jesse Garcia) is a persona non grata at the fiesta. We see him walking the streets of L.A. looking like a thug, complete with his area code tattooed on the back of his neck, and we assume it’s Carlos’ thuggish tendencies that got him thrown out of his parents’ house. When he shows up at the party, underdressed and bearing stolen flowers for the birthday girl, it ends in a fight.

Across the dance floor from pretty, thin Eileen is her and Carlos’ cousin, Magdalena (Emily Rios). She’s less pretty, less thin and less enthused to be here, though she looks forward to her own quinceañera in three months. But she is soon on the outs with her own parents over matters both financial (she wants a limo) and pride-related (she’s embarrassed to be getting Eileen’s quinceañera dress as a hand-me-down) — and then over something far more pressing: Magdalena is pregnant.

Carlos has already been living with his kindly old great-great-uncle Tomas (Chalo Gonzalez), and the two bachelors make room for the newly shunned Magdalena, whose parents don’t buy her story that she never had intercourse with her boyfriend, Herman (J.R. Cruz). Tio Tomas doesn’t judge his young, headstrong relatives. He offers them sage and gentle advice and a loving home instead.

This is a movie about how appearances can be deceiving and how what we expect doesn’t always happen, so I suppose it’s fitting that such a realistic portrayal of Mexican-American life would be written and directed by two men named Richard Glatzer and Wash Westmoreland — from New York and England, respectively. The truth about why Carlos’ dad threw him out, the real story behind Magdalena’s pregnancy, the fact that Tio Tomas is old-fashioned and conservative yet doesn’t reject Carlos’ and Magdalena’s wayward paths — these are the pleasant, happy little surprises about the film, which manages to feel sunny and vibrant even when the characters’ circumstances are sobering.

Glatzer and Westmoreland were lucky to find fresh-faced actors like Emily Rios and Jesse Garcia. As Magdalena and Carlos, their kinship feels natural and their interactions with their peers is honest, sometimes poignant, often funny.

Some of the actors are non-professional, and sometimes the inexperience shows in the way this line or that is delivered. Mostly, though, the cast is thoroughly charming, the characters convincing and sympathetic, the situations unusual but believable. It’s no spoiler to reveal that the film has a happy ending and no exaggeration to say I walked out of it with a spring in my step.

A- (1 hr., 30 min.; R, a few F words, some other profanity, some sexual situations and some sex-related dialogue.)