Venus and Serena Williams‘ murdered sister may have been gunned down in a drug deal gone bad, fear friends.

The tennis champs’ 31-year-old sister Yetunde had been struggling with a weight problem for years. And after failing with diets and weight-loss fads, the 210-pound divorced mother of three may have gone into a drug-infested neighborhood to buy pills that would curb her appetite, say sources.

The ENQUIRER investigated the bizarre street shooting in Compton, Calif., that left Venus and Serena brokenhearted — and got shocking details from the sister of the man who drove the car in which Yetunde was gunned down.

We also learned exclusively that Yetunde was engaged to that man — alleged parole violator Rolland Wormley, 28 — and that she desperately wanted to look more like her buff, athletic sisters.

“Yetunde recently became more determined than ever to drop her extra weight,” revealed an insider.

“A lot of very potent substances are available (on the street) that suppress the appetite better than anything at the drug store — and some friends feared Yetunde was desperate enough to use them.

“The minute they heard details of her death, they worried she was in that neighborhood to buy drugs to help her battle her weight.”

In an exclusive ENQUIRER interview. Rolland’s sister, Carmelle Wormley, revealed: “Yetunde and Rolland were definitely in love. On her birthday in August, my brother asked her to marry him. He would never put her in harm’s way.” Carmelle insists that after Yetunde left with her brother late on the evening of her death, they came under sudden attack.

“My brother said they were driving when a bullet shattered the back window of Yetunde’s SUV.” The bullet hit Yetunde in the torso.

“He drove here, five minutes from where she was shot, instead of the hospital because he panicked,” said Carmelle. “Yetunde was in the passenger seat and there was blood everywhere. She was barely breathing and was slouched in the seat.

“We called 911, and Rolland stayed in the car with her, holding her hand and telling her everything was going to be O.K.”

Police say the couple was involved in a confrontation “with local residents” just prior to the shooting.

But Carmelle counters: “My brother says he didn’t get into any argument with anyone . . . He stopped drinking about four months ago and neither he nor Yetunde did drugs.”

And Rolland’s mom Lorraine told The ENQUIRER: “It was a stray bullet. My son was just an innocent bystander. He loved Yetunde — and we loved her like family.”