In some cases, it's true - Warren Beatty and Shirley MacLaine, Maggie and Jake Gyllenhaal, John and Joan Cusack. Occasionally, a younger sibling outshines the elder - Julia and Eric Roberts, say, or Casey and Ben Affleck.

Most of the time, however, the sibs are a pale imitation. Think of Michael Jackson's kin (with the obvious exception of Janet). Or Alec Baldwin's (excepting no one).

Not that it matters. In our LaToya Nation, we are subjected to them all as they follow one of two standard trajectories:

_ Reality show (yep, that's Stephen Baldwin on "Celebrity Apprentice" and Daniel Baldwin on "Celebrity Rehab"), or

_ Unauthorized biography.

These days Madonna, when not dodging are-they-or-aren't-they rumors about A-Rod, is dealing with No. 2.

"Life With My Sister Madonna," by Christopher Ciccone with Wendy Leigh (Simon Spotlight Entertainment, $26), is a tell-all about "the most famous woman of our time" by "the brother who knows her better than anyone."

The book tracks them from a middle-class Michigan childhood, through the '80s and '90s - when Ciccone served as his sister's backup dancer, dresser, director and decorator - to recent years, when her marriage to Guy Ritchie led to strained relations, vitriolic e-mails and a bitter split with her sibling.

The big dish is that Ritchie is homophobic, and he has coaxed Madonna to ditch her brother, who is gay, and slowly, subtly divest herself of her gay following.

Or so Ciccone suggests.

It's hard to fathom that the woman who made a name for herself vogueing and French-kissing Britney on live TV would really care to do such a thing.

Ciccone presents some evidence - Ritchie's rudeness, Madonna's blasé attitude to it - but nothing that completely swayed this jury.

Much of the book is bland travelogue peppered with zingers.

Her acting: "She has no idea whatsoever how bad she is."

Her singing: "Her moves are, indeed, terrific. Her voice, however, is another matter."

Her "mammoth ego," her "torrent of swearwords," her "great deal of body hair."

None of this is news - except, perhaps, the follicular details but, hey, that was when she was young and not shaving much. Some of the most insightful observations here are quotes from actor Rupert Everett's (better-written) autobiography, "Red Carpets and Other Banana Skins."

Ciccone, as dresser, mops sweat off Madonna's naked body backstage. As dancer, in the days before anyone knew who she was, he works gigs where Madonna promotes her singles (like "Holiday") by singing and bouncing to the track at New York's Studio 54, the Roxy, Pyramid. Even Uncle Sam's in Levittown, "where the crowd just stands still and stares at us."

Ciccone does acknowledge his sister's positive attributes. In interviews, he's said he thinks Madonna has been faithful to Ritchie (though the book doesn't cover the A-Rod scandal).

He gives ample space to her work ethic, her aversion to drugs or excessive drink, and her concern regarding Ciccone's cocaine use. (He says it's not a problem, but given how many arguments they've had about it, it likely has as much to do with their estrangement as Ritchie does.)

An overall sadness pervades the book from the first page, when Ciccone reveals that one of his goals in the book is to finally "define myself and separate from my sister at last."

Really? By writing a book . . . about her?

Sibling rivalry is nothing new. But in today's celebrity-obsessed world, we've raised filial friction to an art form. And for the poor celebrity, a tell-all book by a brother - or a child (Christina Crawford's "Mommie Dearest," Patti (Reagan) Davis' "The Way I See It") - must be the pop-culture equivalent of "friendly fire" - a shot coming at you from a direction least expected.

"I remember feeling a rush of power by association," Ciccone writes of being whisked into club VIP rooms. "I am Madonna's brother. The brother of a superstar. I am so caught up in the magic of my brave new Madonna world that I don't care that I am losing myself, and that working with my sister is now my entire life."

He later writes her, admitting that it's "not an easy task to have people look at me but only see you."

Which brings to mind the book jacket. Madonna's picture is on the front cover. Christopher is on the back. *