It's also not surprising that they may not have believed it for a long time. After all, the press behaved dreadfully towards the sisters when they were younger. I wouldn't blame them for paying no attention and shutting out the media noise.
I imagine it's also a difficult thing to come to terms with - your beloved parents, especially your father, doing terrible things.
When Andrew did his interview, I think Beatrice was in complete denial about the accusations and was convinced he could explain things away. Wouldn't we all want to do that with a loved one?
Their grifting in the Middle East is no worse than what the entire family get up to - which is a problem in itself, but not really something for which they can be singled out.
The whole monarchy is corrupt to a degree, but in this instance I think it's unfair to say the sisters are as bad as their parents just because they might have found it difficult to accept what their father in particular was like.