Are you saying that Martin's characters are realistic, or historically accurate? I would agree that they act in a very realistic human way (sometimes even too much... do I really need to know every time someone drips bacon grease on their chin?), but I would not say that his characters are always socially or relationally accurate to a Medieval setting.
They are not completely accurate, but you can see many norms and realities of the middle ages in the Seven Kingdoms. Even the most outlandish cultures seem to be an exaggeration or modification of some culture found in the world during the middle ages.