Linda Ronstadt rushed from her performance in the "Pirates of Penzance" to the Bottom Line twice last week to hear Kate and Anna McGarrigle sing. The McGarrigles tour about as often as Howard Hughes used to give interviews and Ronstadt -- having had a hit with the sisters' "Heart Like a Wheel" in 1976 -- was obviously hunting for new material. But she was only one of several dozen musicians (including the Roches, who are the commercial, flip side of the McGarrigle sisters) paying professional homage to a pair of singer/songwriters who have spent their late-blooming career marching to a decidedly different and frequently muffled drum.

The McGarrigles, who were in town for two nights at the Cellar Door, have gone about the business of music with an attitude halfway between careless and carefree that has often obscured their considerable talents. Their style is a unique blend of pre'50s folk primitivism and post-'70s social and emotional pragmatism; yet it transcends both influences with an honest romanticism. On top of everything, they sing like slightly soiled angels. "There are different points along the line between zero and . . . Barbara Streisand," says Kate McGarrigle, 35. "We've found a comfortable niche."

Raised in the tiny French Canadian community of St. Sauver-des-monts, the McGarrigles grew up in a tightknit family, surrounded by classical and religious music and heavy doses of French Canadian folk songs. Though they sang semiprofessionally in their late teens, they didn't start writing songs until they were in their mid-20s. They still live in Montreal, from which they venture for brief forays into commercial waters.

Just as they were initially achieving popularity -- their debut album, recorded almost as an afterthought in 1976, ended up on most of the year's Ten Best lists -- they went off the road because of Kate's second pregnancy. Marriages and relationships were dissolving in Fleetwood Mac style, including Kate's long-term partnership with another singer/songwriter, Loudoun Wainwright. "I mean, at one point, there was even talk in a contract about managing any offspring," Kate says.

Both sisters are fiercely independent; despite the critical acclaim given their three Warner Bros. records, they never followed through with the type of career moves expected in the music business. On those rare occasions when they did tour, it was with thrown-together, under-rehearsed bands that tended to fall apart in mid-tour.

More importantly, by staying in Montreal, they purposely avoided New York and Los Angeles, traditionally havens for songwriters of their caliber. Kate remembers Maria Muldaur (who recorded their "Cool Water" and "Work Song") "as a very nice girl who lived up in the country and had a record that became a hit and did the whole star trip. She went out to L.A. and has become almost as obsecure as we are at this point!" As for the Roches, "they tried harder than we did and they'd been doing it since they were 15 or 16. When we were growing up in Montreal, the idea of having a career was unheard of. There were no record companies, so you headed for Toronto or the States."

Having occasionally put French songs on their Warner Bros. albums, they recorded their latest work totally in French, on a separatist Canadian label, at that. "It was doing something," says Anna, 36, pointing out not only that they have served their relationship with Warner Bros. but that their home province is 90 percent French-speaking. "And people in Quebec never heard any of our French songs because they were on English records."

And they have chosen family over fame, each raising two children. "We have certain things that we do in life," says Kate McGarrigle. "One of them is to play music and write songs; the rest of it is raising family. That's a choice that we make. I think it's better having children than a career," she adds. Her children are 7 and 4; Anna's are 3 and 2. "They last longer. It's not as if we want to do one thing and not the other, but it's very difficult when they're so young."

With lessened expectations, they've gone about this brief (10-day) tour with a new approach. "We actually rehearsed for a whole month," laughs Kate. "It's something we never did before. And not at the easiest time either -- 3:30 to 7 p.m. each day. If you have kids, you know that's the worst time: homework, dinner, baths, the whole thing."

The McGarrigles also centralized tour duties with their other sister, Janie, who is serving as tour manager and van driver. Janie also performs as a backup vocalist, as she did as a child and on her sisters' first album. With the children singing new songs at home, and the older sister back in the fold, the McGarrigle family circle will remain unbroken.