Black comedy walks a very fine line. When it's done right it can be both offensive and hysterical, as with "The Loved One," "Dr. Strangelove" or "Harold and Maude."

When it fails, the results may be merely offensive."Sibling Rivalry" is occasionally offensive and only sporadically funny, but its main failing is in being ridiculously convoluted and inconsistent with its characterizations.

Kirstie Alley, hot off her phenomenal hit "Look Who's Talking" (a sequel is coming in December) and her continuing role in TV's very successful "Cheers," stars here as an uptight homemaker who dreams of writing a novel.

Alley loves her husband (Scott Bakula) and is faithful, despite his being neglectful and self-centered. Bakula, a physician in practice with his father, mother and sister (John Randolph, Frances Sternhagen, Carrie Fisher), treats her as if she's a maid and then ignores her when they go to bed.

Alley's sister (Jami Gertz), a free spirit who works in a fish store, urges her to break loose and have an affair, to discover that sex can be fun instead of simply work. Naturally, Alley declines, but later in the day she meets Sam Elliott in a grocery store and throws caution to the wind.

After a day of sex in Elliott's hotel room, Alley is bubbly until she notices that Elliott has died of a heart attack.

Meanwhile, a nerdy salesman (Bill Pullman) goes into the wrong hotel room to install some blinds - guess who's room? - and slips, hitting Elliott's body with a curtain rod. Naturally, Pullman thinks he has killed Elliott.

Of course, Pullman's brother is a policeman (Ed O'Neill), who just happens to be that close to a promotion to chief. So Pullman tries to keep it quiet.

The film progresses - if this is progress - from there to a series of misadventures in the mistaken-identity mode, each more contrived and silly than the one before.

There are some funny, albeit very dark scenes here, primarily a moment in the hotel room when Pullman and Alley plot to make it appear that Elliott has committed suicide.

But there aren't enough chuckles to pull this movie from the comedy dregs.

Martha Goldhirsh's first screenplay is episodic, too talky and only occasionally amusing. This kind of complicated black farce requires a very sure hand, but veteran director Carl Reiner treats the material in a soft, unimaginative manner and most of his expert cast has nothing to do.

Pullman's goofy, slapstick portrayal gets the lion's share of the laughs and Randolph has a few good lines, but no one looks particularly good here. And some scenes, such as Alley's confession near the end of the picture, are uncomfortably awkward.

"Sibling Rivalry" is rated PG-13 for sex, vulgarity, profanity and some gore in an autopsy scene.