Just before the TV season began, during a noncritical ratings period last September, NBC dusted off a sterling 3-year-old mini-series called "The Sophisticated Gents," which had been sitting on a shelf, and dumped it onto the air. Tonight, during a noncritical ratings period after the season has ended, NBC retrieves TV Preview another 3-year-old movie from shelfdom, "Sister, Sister," and unceremoniously airs it.

The two films have this in common: Both are about black middle-class life in America, and both feature large casts of outstanding black actors.

It doesn't take much cynicism to draw discouraging conclusions from the delayed scheduling about the network's attitude toward serious films dealing with the lives of black people, an especially unhappy situation considering the mileage NBC gets out of cartoonish personalities like Gary Coleman on "Diff'rent Strokes" and demeaningly buffoonish ones like Nell Carter on the dimwitted "Gimme a Break."

On the other hand, "Sister, Sister," written by the much-praised poet Maya Angelou--and airing at 9 on Channel 4--has drawn a great deal of attention prior to broadcast, but for all the wrong reasons. The Rev. Donald Wildmon's National Federation for Decency, that big noise from Tupelo, Miss., has urged its members to watch the program to behold firsthand its alleged "anti-Christian" values, even while Wildmon is simultaneously urging a boycott (so far, completely ineffective) against NBC generally because of supposedly scurrilous and sacrilegious program practices.

There's nothing immoral about "Sister, Sister," but, alas for the sake of more than one argument, there's not a great deal recommendable about it, either. The situation in the plot to which Wildmon objects is a case of adultery involving a stern spinster named Carolyne, played by the elegantly beautiful Diahann Carroll, and a fundamentalist preacher with political ambitions, played by Dick Anthony Williams. Wildmon's silly protest--he has not seen the film--is negated by the fact that, according to traditional TV morality, both parties to the affair suffer considerably as a result of it.

But the audience must do quite a bit of suffering as well, because Angelou's script is nothing if not overwrought. The plot is mechanically, and artificially, set in motion when Rosalind Cash, as Carolyne's sister Frieda, capriciously returns to her hometown, where Carolyne lives with another sister, Sissy (Irene Cara, of "Fame") in the big house left them by their father.

Carolyne is a preachy moralist even though she is having the affair with the married preacher, and the drama consists of the other two sisters bringing her to the realization of her own hypocrisy and certain other dour realities. The air becomes not just thick, but suffocatingly smoggy, with accusations, recriminations and melodramatic revelations. It's too much sturm for one household und excessive drang for a two-hour TV movie. Even Eugene O'Neill would have felt parched after such an ordeal.

The performances--all emphatic, rigorous and, in the case of Cash, even radiant--are hearty compensation, however, and some of the confrontations staged by director John Berry do have the power to jolt and disturb. Angelou's dialogue has flavor and authenticity; the best line may be a rather incidental one given to Cara, who tells her sisters, "Some things you know in your mind. You don't feel them in your heart, you might as well not know them."