I don't think she made it up, she talks about it at first being not consensual, that she was heavily drugged and drunk, but then became consensual, and that a part of her knew it was horribly wrong all along. I think the Stockholm Syndrome probably applies in this case as well. Her life has been so messed up, and her father's drug use and poor judgment was well-documented and often commented on. Both her sister and friend and former costar Valerie Bertinelli have come out in support of her, which I don't think they would have done if they didn't believe her.

By the way, I have moved this to The Dog House, as it is by its very nature an upsetting topic.