"...on the cusp of something so profound, that it is hard to put it into words" means that something profound was about to happen at that point in time.
When we can even say for example, when Tom Brokaw was diagnosed with multiple myeloma in 2013, he was "on the cusp of something so profound, that it was hard to put it into words."
Explanation:
It could also mean when someone is about to enter a new experience either in professional life or otherwise. When somebody is overwhelmed by the current situation or position, it may be very difficult for the person to put into words the newly found position. In summary, "on the cusp of something" shows the intensity and profundity of the new situation.
Because ‘Sarah and her sisters has gone’ doesn’t sound right, ‘Sarah and her sisters has been’ also sounds wrong and ‘Sarah and her sisters have been’ doesn’t really make sense
