Many years ago, a lady pushing a pram walked out in front of my car. I swerved, missed the pram but hit her on the hip. I had witnesses too and the police took no action. I remember that she too lay on the ground saying it was her fault. She was in hospital with a broken hip. However, I rang her husband that evening to see how she was and my now late father, ( a solicitor), told me off for doing this as it might have been construed that I was admitting liability.
This was in the late 60's, long before the "no win, no fee" culture, so may be it wasn't a good idea to visit the lady as your SIL did. I don't think they should be worried though.