The kidnaping and murder Monday of a 7-year-old Baltimore boy was allegedly the culmination of a three-month vendetta against his family that began when a neighborhood teenager was ordered to stop seeing the boy's older sister, according to the slain boy's stepfather.

"After that, things started happening," said Willie Godfrey, the slain boy's stepfather. "The windshield of my van was smashed four times, windows in our house were broken, the back door kicked in."

Finally, Monday evening, police say, 7-year-old Mario (Tony) Howe was kidnaped from an alley outside his home, taken to an abandoned water-filled quarry, lowered into the 30-foot deep pad of waters and pelted with stones The boy's body was recovered Tuesday; state medical examiners way he drowned. Charged with the murder were the 14-year-old who Godfrey said was harassing the family and a 15-year-old from the neighborhood. "Both youths were charged as adults understate law.

Before the final tragic act, the Godreys has called police to their home in the Park Heights section of Northwest Baltimore on three occasions between March 7 and last Thursday. Two times they complained the 14-year-old youth was harassing them. The last time, Betty Godfrey, Tony's mother, asked the police, "Does somebody have to be killed before something is done?" her husband said yesterday.

The police, according to spokesman Mike Bass, advised the Godfreys they could file a "juvenile petition" against the youth, which ultimately would have brought him before a judge to face deliquent charges.

No such petition was filed, however, between Thursday and Monday evening, when, according to police, Tony Howe was kidnaped from an alley outside his home. Soon after there was a phone call to Betty Godfrey, a registered nurse and the boy's mother, demanding $300 if she wanted "Tony alive again."

Mrs. Godfrey said she recognized the voice, and police then arrested the 14-year-old who, according to Godfrey, had been banned three months earlier from seeing the Godfrey's oldest daughter. The 14-year-old then implicated the other youth, who also was arrested, police said.

Police conducted an overnight search at an old abandoned quarry where a police diver found Tony Howe's body at 8:30 a.m. Tuesday, on a ledge under 30 feet of water that had accumulated in the pit. According to police, Tony had been tied to a board, lowered into the quarry and pelted with rocks as he struggled to free himself.

Both teen-agers arrested Tuesday were charged with murder, extortion and kidnapping. They were held on bonds of $50,000 and $25,000.

Yesterday, police were saying little about the case for fear of jeopardizing its prosecution, according to homicide Lt. Patricia Mullen who directed the search for Tony Howe's body.

Godfrey, an auto mechanic, was at home with other members of the family who had come to Baltimore from as far away as Norfolk to be with Tony's parents and brothers and sisters. All were searching for answers that did not come.

"I try to figure it out," he said in a telephone interview. "I don't know any of the answers. There are a lot of religious people in my family. My brothers is a minister. No one can explain it. There's no reason for it."

What there was, Godfrey said, was a pattern of harassment that began soon after he ordered the 14-year-old youth to leave the Godfrey home and to stop seeing his 14-year-old stepdauther. "I never did allow her to have company or anything . . . She's too young," he said.

But after his refusal, the vandalism started, Godfrey said. He spoke with the 14-year-olds parents, and on one occasion, the youth's parents paid Godfrey $80 for a new windshield, police and Godfrey said.

"It wasn't the money. I told them I wanted the aggravation to stop," Godfrey said.

The 14-year-old's parents do not have a listed telephone number and could not be reached for comment.