Continuity mistake: When Corrine takes off her blouse, she shows her children seventeen scars on her back that she got from being whipped by the grandmother. Later in the movie, when Corrine is dancing at a party, she is wearing a strapless dress and there are no scars or any signs that she was struck with a whip.
Other mistake: When Christopher is in the bathroom stupidly "dabbing" Cathy's back as she's taking a bath there is a glass block window behind them. The window makes no sense. It doesn't match the era or architecture of the bathroom or the home, first of all, but probably more importantly - what's behind it? Where is the light coming from? It's just this random, standing glass block window with an equally-random light source. (00:34:53 - 00:56:49)
Answer: Short answer, possibly, but we'll never know to what extent if it was. The film is based on the novel written by Virginia Andrews (V.C. Andrews) in 1979. In her pitch to publishers, she said it was not truly fiction. Her claim is the story is based on the life story of a doctor she met when she was younger. The name of the doctor was never revealed. She also wrote sequels to the original book which would have most likely been complete fiction, and there was a prequel ghost-written that most certainly wouldn't have been based on true events.
Bishop73
Supposedly, the doctor told her that he and his siblings were locked in an attic for six years to preserve family wealth. This is according to "an unidentified relative" of VC Andrews. Source: Wikipedia. As you say, we will never know how much was true.