Melvin Jones, who confessed to a D.C. Superior Court jury last March that he killed his aunt and uncle two years ago because they had mistreated his grandmother years earlier, was sentenced yesterday to a jail term of from 40 years to life.

Jones, 29, of 123 I St. SE testified at his trial that he decided during his childhood that he would kill his aunt Virdell Covington, 60 and her husband, John, 68, when he grew up, to punish them for their treatment of his grandmother.

Jones told the jury that his aunt and uncle forced his grandmother to live in the wet basement of their home and cursed her on some occasions within his hearing.

"My grandmother had arthritis and I used to wash her feet because she couldn't. . . ." Jones testified last March 19. "One day my aunt cussed my grandmother. . .I grabbed a knife but couldn't get her. I stuck the knife in the door and told my aunt I was going to get her."

On Sept. 28, 1978, Jones carried out his childhood vow when he and a friend went to his uncle's home at 3828 P St. SE, where Jones said he used a steak knife to kill his aunt and uncle and his uncle's deaf sister, Carolyn Bruton, 64, Bruton was killed, it was said because she happened to witness the other slaying.

Jones' attorney, Grandison Hill, who was surprised by Jones' line of testimony, has attempted since to establish that his client is not mentally competent.

But a psychiatrist who examined Jones at D.C. jail in May testified before Judge Carlyle Pratt yesterday that he found no indication that Jones has a mental disorder.

Judge Pratt sentenced Jones to serve 20 years to life for each of the three murders, with the sentences for the slayings of Virdell and John Covington to be served concurrently.

Jones testified in March that he and the Andre Jones -- no relation -- went to the Covington's home and committed the murders. Andre Jones was found guilty last February of three counts of felony murder.

The two men had planned the slayings for a week, Jones testified. He said they purchased a .38-caliber pistol the night before and also used four knives in the killings.

The government contended that Jones went to his uncle's home to commit robbery.

Assistant U.S. Attorney William O'Malley contented that John Covington was widely known as a "numbers man" and that Covington was known to keep large quantities of cash in his home on frequent ocassions.

Jones testified that his uncle arrived at the house after he had killed his aunt and his uncle's sister. Jones testified that Andre Jones held his uncle on the floor as Melvin Jones stabbed him 44 times.