Mariah Carey's sister claims she is being kept in hospital as a patient for a serious head injury against her will.

Alison Carey, 54, was admitted to a Long Island psychiatric ward at a hospital in New York City after suffering a severe head injury in a mysterious home attack three months ago.

In the assault, Alison - who once worked as a prostitute and is a reported heroin addict - claims the unknown burglar stole jewellery from her apartment and she was struck she woke up with a three-inch gash in the back of her head and injuries to her mouth.

In an exclusive interview with RadarOnline.com, she said: ''I don't remember getting in a car with this person, and I don't remember going to my house. But I remember being at my house and he was taking my jewellery.''

Mother-of-four Alison - who is HIV positive - says she is being held on the mental health ward against her will and is desperate to leave the medical facility and has pleaded for help from her millionaire younger sibling.

She said: ''I need a lawyer to get me out of this hospital because there is no reason to keep me in here for three months ... Please tell Mariah to get me out of here! She must know what's happened to me! I want out!''

Alison has been estranged from her sister Mariah - who is dating Australian billionaire James Packer - for many years and has never been allowed to meet the R&B superstar's children, four-year-old twins Monroe and Moroccan.

Alison has been admitted to the facility before and was a confirmed patient there in 2012.