Read these two threads:
http://www.bikeforums.net/advocacy-s...nds-study.html
http://www.bikeforums.net/road-cycli...tos-bikes.html
It is hard to actually pin down how dangerous bicycling is. However, a lot of people believe that it is also HEALTHY, and the health benefits may more than offset the inherent risks.
That being said, it never hurts to look at the accident and try to make things better. Was it a fluke accident, or did the sister do something to cause it? Will it happen again, or what can be done to prevent it from happening?
One might consider a recumbent trike? Off-street riding?