She should tell the truth. Tell them what she remembers, and nothing about what she does not. Depending on the nature of the assault and the motivations, it could very well be that the brother has a problem and needs to be treated, punished, and/or removed from the possibility of harm to others. If he has an "illness" or abnormal predilection, he could be a risk to others and this must be addressed! Your fiance would not be doing anyone any good by lying or avoiding the police.
She should also consider seeking counseling for what happened to her. Her shifting the blame to their parents is not healthy. While her parents may have been partially at fault, perhaps, for creating some circumstance that may have permitted this to occur, the fact that they did not provide "sex education" to their children is a cop out and not an excuse for the brother's behavior at all.
So, my suggestion would be that she tell the truth as best she can recall. AND, she should seek counseling so that she might adequately come to grips with what happened.

