MARIAH CAREY is on top of the world preparing for the birth of her twins – a boy and a girl – but her down-and-out sister ALISON – a mother of four – is so strapped for cash she’s selling herself as an "adult escort," The ENQUIRER has learned exclusively.

Alison Carey Scott – a former drug addict who’s been arrested twice for prostitution and is also HIV positive – is shopping herself on the Long Island, N.Y., edition of,a free internet classified service used by many hookers.

While the adult entertainment ad touts a 36-year-old sexy blonde named "Denise," The ENQUIRER learned the ad is actually for 49-year-old Alison, who charges $250 an hour for house calls. The picture is fake, natch.

"You have no idea how badly I need cash right now," a distraught Alison admitted to The ENQUIRER.  "But I’m not having ‘sex for money’! I provide personal companionship and erotic body rubs."

In an emotional and candid conversation, The ENQUIRER spoke to Mariah’s sister when she showed up Jan. 21 to meet a male business traveler in room 431 at the Hilton Garden Inn hotel in Ronkonkoma, N.Y.

Dressed in jeans and a baggy sweater, Alison said: "I’ve been forced to do this because my family refuses to help me financially.

"For weeks, I was driving in a vehicle with practically no brakes. It was unsafe, so I reached out to my family for help, but they basically ignored me. I wish I didn’t have to make a living by hooking up with strangers on the Internet, but sometimes I feel like I don’t have any other choice."

Alison’s pathetic plight stands in sharp contrast to her sister Mariah’s fairytale life. The 41-year-old, five-time Grammy winner is expecting twins this spring with her 30-year-old husband Nick Cannon.

In 2000, Alison tried to find a publisher for her explosive tell-all book about her famous sister – reportedly dubbing her a "vain and heartless multimillionaire," but Mariah’s legal team allegedly put a stop to the publication.

Things continued to go downhill for Alison, who was busted for soliciting prostitution in June 2005 after offering to have sex with an undercover officer for $250. It was her second prostitution arrest within a 10-week period.

Alison, who had her first child at 15, has battled HIV since the 1990s.

In 1995, the sisters’ mother Patricia Carey – fearing for the safety of Alison’s then-7-year-old son – reportedly triggered a custody battle by refusing to hand him back to her troubled daughter after a family visit. A court later awarded them joint custody.

Although the sisters are still at odds, mom-to-be Mariah has reportedly paid for Alison to go to rehab multiple times.

"Regardless of how it might appear, I am trying very hard to turn my life around," Alison told The ENQUIRER. "I’ve been clean and sober for 14 months. I attend drug and alcohol support meetings, receive counseling and have a sponsor to lean on. I’m also enrolled in college, which I hope will someday provide a better life and financial independence.