God bless the child that's got his own."

- Billie Holliday

Marvin Isley is in his early 20s - the baby of the family - and a millionaire a couple of times over. At 10 a.m., just off the plane from Roanoke, his barrel chest (a family characteristic) is encased in rhinesetone-studded black velvet. The other diamonds are real: One bracelet spells out "Marvin," the other "Prince"; the watch face is eclipsed by the diamond casing, the ring is bigger than the joint it covers, and the lion's head around his neck, his astrological symbol, has glinting eyes. More gems are crammed into a box in his carry-on bag.

Marvin is one of the Isley Brothers, the hottest clan in soul music, and a family that believes in getting its own. They've been at it since their first big hit, "Shout - Parts I and II" in 1959. Among their other singles were "Twist and Shout" in 1962, "It's Your Thing" in 1973 (the first which included all six brothers) and the current "Take Me to the Next Phase."

Their latest album, "Showdown," sold its first million copies in four weeks and has already doubled that. They have their own record label, T-neck, several publishing firms, shopping malls and buildings from Atlanta to Houston - plus uncounted smaller investments, antique cars (for example, a customized 1936 Auburn with mink seatcovers), musical instruments and a fortune in wardrobe.

Probably the only comparable entity in popular music today is the Osmond family: a family of brothers which later included the younger siblings; a family in complete control of all music-business assets; an extremely clean-cut group held together by religious conviction as well as by natural affection. The major difference is that the Osmonds have willing to allow one or two personalities - Donny and Marie - to stand out in front of the rest, while the Isleys are adamantly united. "We're a family before we're a group," insists Marvin. It's the perfect blend of music and moxie.

"Show business is two words," says Marvin Isley, "and a lot of people forget about the second word. If you just get up there and play, the business never gets done. That's what our folks tried to teach us; whatever kind of job you do, you ought to be able to get something out of it."

At the Capital Centre Saturday night, the Isleys demonstrated their respect for both the playing and the business. Their set was full of seeming explosions - including three or four giant flashpowder burns - but under complete control at all times. And it's nearly all their own sound - no horn section or extra percussion, just the six Isley Brothers and three friends on drums, backup keyboards and thythm guitar. (Only the brothers play on the albums.)

Respect for the Isleys has served them well, spirtually as well as financially. "Musicians are funny; they have different temperaments," says Marvin Isley. "By the six of us being brothers, we can be honest with each other.

"You're on the road a long time, and you have to see each other every day. We're fortunate to have our own label, our own manager, our own producer. We only go out on tour when we're mentally and physically ready . . . when we go out on the road, we expect to have everybody ready for just that."

Actually, although even Marvin calls the six brothers, only five - Kelly, Rudolph, Ronald, Ernie and Marvin - are Isleys. Chris Jasper is Rudolph's brother-in-law, but he grew up around the corner and was assimilated into the family so long ago that the distinction has been lost. The original fourth Isley - their father wanted four sons to replace the Mills Brothers - was killed in a childhood accident.

And there was once another notable member: Jimi Hendrix, whose legacy was an indelible imprint on guitarist Ernie Isley that powers the group today. Dressed in satin pants and yellow chiffon butterfly sleeves, with a scarf tied around his forehead, Ernie bears an eerie resemblance to his idol and predecessor, especially when hit with a shimmering yellow spotlight. The solos recall not only Hendrix's innovative bending of tones and notes, but his physical style; Ernie Isley arches and contorts over his instrument, fires point-blank at the audience with it, even plays it with his teeth as Hendrix used to do.

"Ernie and Chris and I really admired Jimi and the way he played," says Marvin Isley. "Ernie was just getting into guitar (when Hendrix died in 1971) - it was a real shock."

Hendrix's influence and the jazz training of the younger Isleys have moved their music from Motown to soul that teeters on the edge of funk. On stage, the group divides itself into three sets of three: Kelly, Rudolph and Ronald, the vocalists; Ernie, Chris and Marvin on guitar, keyboards and bass - more prancing and peripathetic than their elders - and the three un-Isley musicians well to the rear of the stage. The sound splits up similarly, with the beat raging below the Motown-mellowed vocals.

"We're very particular about the voicing of the instruments and the voices," says Marvin Isley, talking business again. "We keep that Isley Brothers sound, the basic track first, so that when you go to the concert you hear just what you hear on the album."

It's been a best-selling combination - the flesh of the music and the flash of the sequined costumes - and there are other artists who would like to get close to the Midas touch. The Isleys have been invited to produce artists ranging from Sly Stone and Patti LaBelle to the Jacksons and Dionne Warwicke, although they haven't yet found it feasible. They are currently building their own recording studio, which may allow them to expand beyond family.

They don't forget, however, that success can have a withering effect. "We don't try to stay on the road till we lose all perspective. Like when we're on the road, everybody treats us like, 'Wow, the Isley Brothers!', and some people get caught up in that bull. Well, we try to keep publicity at a level we can handle. You know it's funny to know other people (entertainers) personally and then see them as performers - seeing them on stage and then off . . . it's like a mirror."