When Craig Whitney allegedly gunned down a Southwest Philly man last summer, he unknowingly set in motion a chain of events that may have led to the slaying of his cousin almost a year later.

Or so it seemed yesterday, when, police said, two relatives of Whitney's alleged victim opened fire on Whitney's family, killing his cousin, Nyesha Whitney, and injuring two others.

Police and relatives say Nyesha, 23, a mother and college student, was a bystander who got caught in the middle of a long-standing clash between two families.

"Nyesha was a hardworking mom, a beautiful person with a beautiful smile," said one relative, who asked not to be named. Nyesha had a son who was almost a year old.

"She never had an enemy. She was loved by hundreds of people and this is a terrible loss for family and friends," the relative said.

Craig Whitney, 20, of Florence Avenue near 58th Street, in the Kingsessing section, was arrested in Southwest Philadelphia March 4 in the Aug. 10 slaying of Anthony Jackson, law-enforcement officials said.

According to police, Whitney shot at Jackson's car as Jackson drove along Chester Avenue near 54th Street.

Police believe that Whitney had targeted Jackson, of Cecil Street near Chester Avenue, but didn't say why.

Since then, altercations between the two families have been commonplace, said the relative.

In a courtroom yesterday, where Craig Whitney is being tried for the murder, emotions ran high as Whitney's and Jackson's families came to blows, temporarily halting proceedings.

"It got hostile, everybody started fussing," said the relative.

"It's been going back and forth like that for a while."

Later in the day, about 4 p.m., dozens of people, including Nyesha Whitney and several small children, congregated outside of her parents' house on 53rd Street near Chester Avenue.

Two men approached them near the front porch, opened fire and took off, police said.

Whitney was struck once in the head, her 71-year-old great- uncle, identified by her relative as "Uncle Archer," was hit in the hip, police said.

A 19-year-old friend of the family was struck in the hand, police said.

All were taken to the Hospital of the University of Pennsylvania, where Whitney was pronounced dead at 4:15 p.m. The two other victims are listed in stable condition.

At the crime scene, investigators marked 16 shell casings that spanned the width of several residences on both sides of the street.

Clusters of crying neighbors and relatives clutched one another as they watched investigators step around spots of blood that dotted the steps and sidewalk.

Whitney was a star basketball player at Edward Bok High School, 8th and Mifflin streets, and was featured several times in the Daily News as a high-scoring guard during her junior and senior years.

Recently, she had been on maternity leave from her job at Brightside Academy.

She was a semester away from getting a degree in early-childhood education at Community College of Philadelphia, said a family member. *