IN a heartbreaking case of sibling rivalry, BILY JOEL’s daugh­ter is upset with her supermodel mom CHNRISTIE BRINKLEY for appearing to play favorites with the kids.

Singer Alexa Ray Joel, 27, feels that Christie did little to help her break into showbiz while she’s doing everything in her power to turn her gorgeous 15-year-old half sister Sailor into a fashion model, a source says.

Alexa, who’s admitted to battling depression, made headlines in 2009 when she attempted suicide – reportedly following a breakup with a longtime boyfriend. She later claimed she was not trying to kill herself.

“She has always felt insecure about her looks,” a source told The ENQUIRER. “And now Christie is making a huge deal about turning Sailor into a star. Alexa feels hurt and isolated from her mom.”

Christie has become a cheerleader for her youngest child, declaring on Facebook: “I am sooooooooooooo EXCITED for my daughter Sailor! Her first fashion spread ever came out today!”

In contrast, Alexa, who once appeared in a Prell shampoo commercial, flopped as a model and is now concentrating on a music career.

Alexa is the only child of 59-year-old Brinkley and Joel, 64, who were married from 1985 to 1994. Sailor is the supermodel’s daughter by her fourth husband, architect Peter Cook. Christie also has an 18-year-old son, Jack, by third hubby, oil heir Ricky Taubman.

Alexa is now struggling to escape from her half sister’s shadow, says the source. “But she’s sensitive, and it doesn’t help that Christie is showering Sailor with praise so publicly.”