The Everly Brothers have it.

The Brothers Four don't.The Lennon Sisters do, but not The Saliva Sisters.

We hear a lot more about sibling rivalry than sibling harmony these days, though one is just as real as the other.

Sibling harmony is a musical term that refers to the tight -- almost inseparable -- vocal harmonies generated by members of the same family.

The Brothers Four didn't have it because they were only fraternity brothers.

But Phil and Don Everly are blood. Their voices blend so well you can't tell one from the other. And they produce the kind of sweet sound that audiences have enjoyed for centuries.

"There's no question in my mind that when members of the same family sing together, something intangible happens," says Craig Jessop, associate director of the Mormon Tabernacle Choir. "The Osmonds, the Lennon Sisters, there's something magic in their music. I can't help but feel it's a spiritual element."

Dr. David Power, vocal performance chairman at the University of Utah, also sees that spiritual component. For him, all singing is communication, spirit to spirit, and family closeness simply enhances that unity. But Power also sees several physical reasons sibling voices blend so well.

"With family members," he says, "noses are alike, ears are alike. So it stands to reason voices will be alike. I know when my sons answer the telephone there's often a question about who's speaking. We all sound the same when we sing, too."

One reason, says Power, is that resonance of voice comes from sounds being enhanced in various chambers: the oral cavity, the nasal cavity, the pharynx.

Like the box of a guitar of a violin, parts of the face and head serve to amplify and color tones. Just the size and shape of your mouth changes vocal quality. So members of the same family produce the same "timbre" because they

have the same facial structure.

What's more, personality may come into play. People who share the same genes and grow up together often approach life -- and the arts -- with similar attitudes and temperament.

Music history is filled with such examples.

Will any folk group ever achieve the oneness of purpose and sound that June Carter Cash and The Carter Family were able to generate? Probably not.

And what of the three Wilson brothers and the famous Beach Boy sound, the Andrews Sisters, The McGuire Sisters and one of the hottest harmony groups on the circuit today, the Osborn Sisters, who sing as the group SheDaisy?

They all meld together perfectly. And that vocal unity often holds up even when rifts appear in family relationships. The Everly Brothers had a spat and didn't speak for years. They still live is separate parts of the country and seldom get together. Yet when they take the stage, their Kentucky childhood and the years of working with their father, Ike Everly, come to the fore and create a sound that's as tight as twin fiddles.

The same can be said of the Martins, one of the hottest groups in Southern gospel music. Judy, Joyce and Jonathan not only shared hardships in their younger years but shared the fervent, zealous Christian faith of their parents. When the trio busts a chord on "What a Friend We Have in Jesus" or "Christ the Lord is Risen Today," it sounds like one voice and 50 voices at the same time.

No other harmony group sounds quite like them.

Locally, of course, the kings of kinship harmony are the Osmonds. In the early years the harmonies of the four boys charmed national audiences on the Andy Williams Show. (Williams, of course, was a "sibling singer" himself with The Williams Brothers and took a shine to these siblings of a new era.)

In later years the duets of Donny and Marie and even the vocals of the Osmond cousins have kept the family in the spotlight.

Something in the family sound has been irresistible.

"Siblings who sing together a lot develop a very special sound that comes simply from singing together," says choral director Dr. John Cooksey of the University of Utah. "Their vibratos are often similar. And the blend of voices sets up overtones and creates harmonies that are intriguing. It is a pleasing and warm sound."

And, judging by the the Top 40 over the years, also a lucrative one. The names of brother and sister groups fill the pop music history anthologies.

In fact, as long as they keep making siblings, siblings will keep making harmony in order to make a living.

And that fact should be music to every music lover's ears.