I'm sorry your sister was attacked and I hope she recovers fully.
I don't know but I would imagine if you choose to walk through a field of livestock you take your chances, I don't think you can sue.
The cows will be after the dog not normally the human. In a situation where you think you are about to be stampeded it is recommended that you release the dog. The cows will go after the dog, which can probably out run the cows and divert attention from you giving you chance to make an escape.