It won't be until, say, album No. 3, at least, that you read a story about the Belle Brigade that fails to mention their lineage. So let's dispense with that right off the bat, shall we?

The Los Angeles sextet is fronted by Barbara and Ethan Gruska, the children of composer-songwriter-producer Jay Gruska (who has a long list of TV, film and songwriting credits) and the grandkids of composer John Williams (you know, that John Williams — the one who scored most of your childhood film memories, from the Indiana Jones movies to Superman, E.T. and those obscure art-house flicks about Luke Skywalker and Darth Vader).

So what have those bloodlines instilled in the Gruska siblings?

“Basically,” Barbara says, “to not be afraid to suck.”

Songwriting, they explain, is a science based on trial and error, its success determined by how much elbow grease you apply to the trials, and the gumption with which you face the errors. “It's not really magical,” Barbara says. “It's work.”

And in the making of their self-titled debut album, due April 19 on Reprise Records, they found themselves siblings-in-arms — six and a half years separating them in age but uniquely joined by a sense of purpose.

“Nothing ever really came in a minute,” Ethan says, “but we weren't afraid to keep at it for a really long time. Sometimes when you don't get something right away, you want to give up, but we pushed each other to keep at it.”

The album belies that intensity; it's a buoyant, tightly wound take on 1970s folk-rock that makes melodies and harmonies sound effortless. It's delivered with such cherubic exuberance, you'd think the Gruskas had never hit a bump in the road, let alone taken a wrong turn.

In person they're like that, too — they look as if they just walked out of an episode of The Brady Bunch (well, except for Ethan's 21-year-old scruff), they smile almost chronically and they finish each other's sentences with remarkable politeness. Through their blood relations and extended musical family — besides their family friends, Barbara has established herself as one of the most beloved drummers on the L.A. scene, having (wo)manned the kit for myriad acts, including Jenny Lewis and Inara George — it's the stuff of made-for-TV movies. Oh, there's one of those in the family annals, too.

Their mother, Jenny Williams Gruska, had a stroke while giving birth to Ethan. Doctors were not optimistic, but “she made a one-in-a-million recovery,” Barbara says, and the story eventually was told in the 1994 TV movie A Time to Heal (for which Jay wrote the score). Their mother's experience is referenced in Belle Brigade's “Lucky Guy,” one of Ethan's songs for which his sister, ironically, co-wrote the lyrics.

“The song talks about his personal experience,” Barbara says, “but I'm so intimately involved in his personal experience I can write lyrics through his mouth. It's unique, to know everything about somebody's past, and to have the luxury of his trust so I can write about it.”

Of course, it was not always so. As kids, Barbara says, “We picked on each other hard. …” Ethan: “Between the age difference and the fact I was a pretty rambunctious kid, we knew how to get to each other.”

Barbara was a drummer from the get-go, but Ethan seemed preternaturally talented as a performer. “He was 10 years old and more popular at my high school than me,” Barbara says, laughing. “He had the lead in the high school play.”

Ethan mellowed as a teenager, and by the time big sister settled in as a gig monster for local bands, “she was my musical idol,” he says.

Ethan had been writing songs since his early teens; Barbara started later. “Just a thing I did by myself at night,” she explains. In the spring of 2008, the Gruskas, joined by friend Bram Inscore (a song- writer/multi-instrumentalist who was Beck's touring bassist), went to the Hangar Studios in Sacramento to record some of their songs. “That's when we decided to merge,” Barbara says.

An ensuing trip to Israel — which may or may not have included high jinks such as swiping a rabbi's guitar and ditching the tour to play songs — solidified the notion. Later that year, at their father's birthday party, they played a couple of songs, and afterward a friend of the family, producer Matthew Wilder (No Doubt's Tragic Kingdom), offered to help.

Progress was sporadic because of Barbara's tour commitments, but eventually the album was completed. “Working with Matthew was instant family,” says Barbara, who drums on the album but mostly sings and plays guitar live. “We could be goofy, we could be anal, we could be weird, and nothing would be uncomfortable.”

Their extended family played roles, too — young guitar virtuoso Blake Mills contributes, as does keyboardist Aaron Arntz (an alum of Zappa Plays Zappa) and B-3 legend Mike Finnigan (who played with Jimi Hendrix on Electric Ladyland, among a long roll call of credits). “He was our neighbor growing up,” Barbara explains.

Jay Gruska was there, too, in spirit.

“When I was 19 he told me, 'You're not going to be able to do this unless you say yes to every single gig and practice your ass off,' ” Barbara says. “It scared the shit out of me, but I did that.”

Belle Brigade perform April 9 at House of Blues.

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