As with all these books, there are lots of opinions bandied about by people who either haven't read them, who churn out 'quotes' or opinions that they have heard before, or who disagree with the concept so much that they won't accept that they do work for some people.
There are also several valid criticisms of the methods.
They don't work for everyone.
Most people who say the book was helpful, don't follow it to the letter, and arguably they are the ones who will get the most out of it, by picking out the useful bits and adapting it to their situation.
IMHO following the BW methods won't be detrimental to bfing in most cases, but may not suit everyone.
No-one will know until the baby arrives, and it is up to your sister to decide.I'm sure she will be very grateful for your knowledge and support.
But don't write the BW off until you've read it. And whatever you think, it has sold millions of copies worldwide, many many people swear by it, and many of those mums will have breast fed successfully.
I didn't follow it by the way, but I have read it