My views on stepping in to help mimic many of the idea already put up here. I do not believe that the responsibility resides solely in your martial arts training, but in your moral views as a human being. A good person will step in to diffuse a situation in whatever way they have the ability, be it physically blocking the attacker from a continued onslaught, interupting the confrontation verbally and try to talk the assailant down, or calling the police and stepping in to perform first aid when the conflict has ended. Because of our training and confidence in our abilities, I do believe that make a martial artist more prone to stepping in to halt a conflict, but I don't believe that it increases our obligation any more than the next man.
A good man will do what he can, when he can. A bad one will think only of himself.
In reference to it being one of my loved ones being attacked, I would not care if it was a martial artist stepping in or not. So long as the ones I care about are protected and the situation dissolves as painlessly as possible. Just because you have training does not mean that you can handle every situation, but conversely just because you don't have training does not mean you cannot handle those same situations. All training gives is physical tools, and a strong, moral mentality.