You didn't give a lot of specifics, so I will give you a general answer.  First year law students learn that, for an injured party to recover on a negligence claim, they must show that the defendant owed a duty to the plaintiff, that she breached that duty and, as a result, the plaintiff suffered damages.  Essentially, in this context, a duty is a legal obligation that the law places on one party toward another to act in a certain way (or to refrain from doing something).  So, to your original question, when we are performing some action (like driving a car), we all owe a duty to those around us to act as a reasonably prudent person would under similar circumstances.

However, that does not mean that a duty exists in all situations.  If there is no duty, there can be no liability, even if a different action by the person in question would have lead to a far better result.  As an example, in most jurisdictions there was no common law duty for an bystander to assist an accident victim.  Accordingly, the victim (or surviving family members) could not sue the bystander for failing to give assistance.

So, whatever the fact pattern, your brother will need to convince the finder of fact that the defendant breached his duty of acting like a reasonably prudent person under the circumstances.  It sounds complex, but it really comes down to common sense.  Follow this link for more information about the elements of a personal injury claim in MA.