unfortunately there is not really any way to know. The awful thing is that children's memories are malleable enough that even if the step-daughter were lying, she could have come to believe her own lies by now (or the lies of her mother, or the content of a nightmare, or whatever).
There is also the equal possibility that this is true, and Allen is a monster walking among us. We -do- know that he married his former stepdaughter, right? So at the very least he is a monster in legal ways.
It's easy for me, because I have never liked his movies very much. Now maybe people will stop trying to make me appreciate them.
The thing about Allen is that even in his movies he is a creep. A guy who pretends to be sensitive and nice but is using his "sensitivity" to manipulate people into giving him what he wants. The portrait he paints of himself is absolutely that of an emotional abuser, although obviously movies are an exaggeration of reality.
Reading that article informs me that Midnight in Paris was his. This means I enjoyed one of his movies, which I didn't think I had ever done, despite several viewings of Annie Hall and Manhattan.