Sultry songbird Mariah Carey (search), who's back on top with chart-busting hits, is now confronting a heart-wrenching new worry: whether she's become an enabler to her HIV-positive, prostitute sister.

Last month's arrest of the curvy crooner's older sister, Alison Carey Scott (search), on charges she solicited an undercover cop for sex on a boat at Huntington, L.I.'s West Shore Marina, hardly shocked the Carey family.

It was Alison's second prostitution bust in 10 weeks.

And the 44-year-old mother of four — who's been HIV-infected for at least 15 years — has been turning tricks off and on since the early 1980s, largely, friends said, to fund her longtime drug addiction.

While Mariah has repeatedly paid for her sister's efforts at rehab, at this point, "she's looking for Alison to want to help herself," a spokesperson for the singer said.

"Mariah can be there with love and support — but at what point do you not enable someone any longer?"

For a heartsick Mariah, 35, Alison's arrests made painfully public the illicit lifestyle from which she has tried to save her sister after a hardscrabble Long Island childhood.

"Most everyone else in the family has had it with Alison," said a source close to the family. "But Mariah isn't one to give up on people.

"Unfortunately, Mariah is having to learn that you can't help someone who doesn't want to be helped."

Alfred Carey, an engineer of Venezuelan-African descent, and his wife, Patricia, a singer of Irish descent who sometimes performed with the Metropolitan Opera, had struggled financially while raising Alison, son Morgan and Mariah in Huntington.

The Careys divorced when Mariah was 3. Mariah stayed with her mom, moving 13 times as a child while Patricia struggled to make a living. Alison went to live with Alfred.

By 15, Alison was pregnant; by 16, married and a high-school dropout. She soon divorced the father of her eldest son, Shawn, and started hooking to support a burgeoning drug habit.

In 1988, Alison had another son, Michael. When he was a toddler, she contracted the AIDS virus.

Mariah, meanwhile, moved to New York City after high school, intent on launching a singing career.

Brenda K. Starr, a singer for whom Mariah sang backup, made sure the teen's demo tape got to star-making music mogul Tommy Mottola.

By 1993, she was a star and married Mottola, with her sister as a bridesmaid.

As Mariah's star rose and made her a multimillionaire, Alison's drug use continued — and so did her prostitution gigs, for which she charged $200 to $300 an hour.

"Alison has been in rehab at least 10 times — many times financed by Mariah," a source said.

By 2002, when Alfred Carey got sick, the family wanted nothing to do with Alison — who by then had had two more children, a daughter, 4, and son, 6.

Although Mariah, having divorced Mottola in 2000, was fresh off bitter breakups — with both her beau Luis Miguel, and her label, Virgin — and recovering from a well-publicized breakdown, she decided to reach out to her sister.

"Mariah, especially, wanted a reconciliation. She thought it was important to try and have something good come out of an awful situation," said a source.

Alison no sooner had a $1 million inheritance from her father — who had become a high paid aeronautical engineer for Grumman before his death — than she began spending.

"When she got the money, she would spend a week and a half living at the St. Regis, racking up $15,000 tabs," one family friend said.

"It was very painful for the family to see her burn through what Alfred had worked his whole life for."

Recently — "around the time the money ran out," according to a friend — Alison went back to her old tricks.

For more news, entertainment and sports coverage, click here for NYPost.com.