SIBLING HARMONY. Born just 18 months apart, the Morales sisters are like a couple who've been together long enough to know each others' thoughts. The singing, guitar-playing and songwriting duo have braided their voices together since they were 5 and 6 years old.

"We had to work it all out, but I think we're successful at it," says Lisa Morales from her home and production studio in San Antonio. "It's like working a marriage. We run a business together, but when we've both got PMS, it's crazy!" she exclaims.

I can hear Roberta Morales cackling in the background, adding that it was a nice, long 18 months before her younger sister arrived on the scene.

Sisters Morales is the official name for their band, cooked from an eclectic stew of Tex-Mex music, boleros and rancheras and their own contemporary, country, rock and Mexican-tinged songs. With a couple of CDs produced on their own label, Luna Records, the sisters finally came out with an all-Spanish album last year. It was a long-held dream and not surprising, considering they'd been pushed on stage at a young age at Tucson's La Fuente Restaurant.

"Our dad would not only encourage us to sing along with the house Mariachi band, but to teach the guys a thing or two, and then perform the songs for the patrons. But Dad would also make us fold our hands in our laps at the table when the mariachis would come 'round. We couldn't eat while they were singing. It was out of respect for the music," says Lisa Morales.

Music was king in the Morales household as well. Growing up in Tucson, the girls heard plenty of Mexican music. But there were other influences. Their grandmother and great aunt played classical music. Dad would bring home Johnny Cash and Sinatra. Mom would spin all kinds of world music. Their older brother blasted the Beatles or Hendrix or Buffalo Springfield.

"You always go back to where you're from," says Morales of this mish-mash of musical influence. "We've never not done Mexican music, so it was natural for us to do a whole CD in Spanish. People say that when you sing in Spanish, it comes from your soul. It's a funny thing--I can sing higher in Spanish than in English."

So why'd they leave Tucson and their roots?

"I think sometimes it's hard to do what you want to do with your family around. They know everything about you."

The sisters went their separate ways for a while. Roberta moved to California, Colorado and Dallas. Lisa trotted off to Houston. They met up in San Francisco.

"We tried to do music there, but it only lasted nine months," explains Lisa Morales. "Here I was, playing music five nights a week in Texas, succeeding at it, but up in San Francisco, it was so difficult to make a living playing music."

They landed in San Antonio, a place Morales says feels like home.

"I'm a little disappointed that it's not more musical, like Tucson, despite folks like Flaco Jimenez being here. But Austin is only an hour away," says Morales of the southwestern music Mecca.

Their 1999 album, Someplace Far Away From Here, has a country twang, with Morales' husband David Spencer's electric guitar work prevailing. Lush renditions of the sisters' harmonizing interweave their own compositions--classics like "Sabor a Mi" and "Historia de un Amor" from the '50s. They sound a lot like Tish Hinajosa or someone from the Carter/Cash clan.

"We played in Denver a while a go, where they billed us as the love child of Bonnie Raitt and Los Lobos! If we could throw in some genetics from John Hyatt, that would be an ideal description," says Morales.

A decade ago, RCA attempted to squeeze the sisters into a Latin Judds template.

"It just didn't fit. We won't be going back to RCA unless they get some different ideas about music," Morales says definitively.

That was a difficult time for the sisters, particularly Roberta, who battled cancer. Touring would have been impossible. Now healthy and back on the road, their identity is solidly independent, secure enough to produce the all-Spanish Para Gloria, their latest album dedicated to their mother and her influences on their lives.

"We wanted to do this CD while she's still alive and can enjoy it."

The sound is immediately different from their mixed-language CDs.

"Well, we have a larger group of musicians--three violinists, an accordionist (and) someone playing vihuela and guitarrón. We'd love to bring that show to Tucson, as the nine-piece band has a whole different flavor from our five-piece group. It's really lush."

Future projects include a new, more rocking English album and then another Spanish album, then maybe even one in Portuguese with Brazilian influences.

The sisters are excited to play the El Casino Ballroom rather than the Rialto or Plaza Palomino, where they played a couple of years ago. Being part of the generation of Mexican-American families who lived in Tucson--during the era when the ballroom wasn't as vital as it's becoming now--adds a new twist to their show.

"We never went as a kids--the roof was caved in. But now we're really looking forward to getting up on stage there."