"Sibling Rivalry" is puerile slop, a ghastly comic catastrophe about a neglected doctor's wife who accidentally sleeps with her brother-in-law. Appalling in every respect, this gauche Carl Reiner-directed farce brings out the most irksome qualities of a cast headed by the gawky girly-woman Kirstie Alley.

Alley plays the sexually repressed Marjorie, a put-upon homemaker who is married to Harry (Scott Bakula), the youngest sibling in an obnoxious family of physicians. Her abrasive sister (Jami Gertz), a poor excuse for a zany bohemian, suggests that Marjorie, a virgin bride, have an affair so she can at least contrast and compare.

As fate would have it, that same day she meets a walrus-faced man (Sam Elliott) in the grocery line, and before you can say "paper or plastic" is swept off for an afternoon o' passion. Coyly, Reiner's camera follows a trail of discarded lingerie, while off-screen Marjorie moans and shrieks, getting into the total grooviness of lovemaking: "Oooo. This is delightful. Oh, in-a-gadda-da-vida, oh God, oh God, rock-and-roll." Five orgasms later, Mr. Walrus Face dies from amatory excess, thereby setting off a chain of painfully graceless comic moments.

Bill "Bad Timing" Pullman, who is to comedy what an earthquake is to a rising souffle, figures prominently as a doltish vertical-blinds salesman who helps Marjorie make the death look like a suicide. This entails stuffing stool softeners and cramp tamers -- which Marjorie keeps handy in her purse -- down the corpse's throat with a pencil. Then they find he is still wearing a condom, and argue over who will remove it. Tee hee.

As written by Martha Goldhirsch, "Sibling Rivalry" takes a squeamish, phony-'50s approach to the human condition. The best thing you can say for this chirping inanity is that the characters practice safe sex.

Sibling Rivalry, at area theaters, is rated PG-13.