Angelina Jolie and Brad Pitt's relationship has been rocked by shocking new sex and drug allegations in two forthcoming biographies – including claims that the actress once slept with her late mother's lover, and details of her struggles with heroin.

Amid reports by journalist Ian Halperin that Brad and Ange are being torn apart by "irreparable tensions" caused by her wild lifestyle, the latest cheating allegations have stunned Brad and reportedly even alienated the actress from her brother, James Haven.

Although Angelina has long criticised her father, actor Jon Voight, for his many affairs while married to her mum, Marcheline Bertrand, respected biographer Andrew Morton is set to accuse her of a shocking secret betrayal.

"Marcheline had a live-in boyfriend whom she was very much in love with, but Angie slept with him when she was 16 and barely out of school," a publishing source told Britain's Now magazine. "Her mother found out and ended her relationship with the man."

While Angelina's rep had declined to comment on the claim at the time of press, the star – who denies she wrecked Brad's marriage with Jennifer Aniston – has often said she'd never be involved in cheating.

"To be intimate with a married man, when my own father cheated on my mother, is not something I could forgive," she once said. "I could not look at myself in the morning if I did that…"