It is possible what she is saying is true but not the context – has he ever helped her get dressed or anything? You need to have her evaluated by a professional to determine what has occurred, not the Internet.
First and foremost: therapy for ALL of you. Ask her questions, but don’t grill her for answers if she doesn’t want to answer them. Be there for her, support her, let her know that you’ll be there for her no matter what. Never, ever admit to her that you have the slightest doubts in what she’s saying.
As for the lie detector test, there are some states in the US where lie detectors are not applicable in court because of how unreliable they can be.