My mother told me once that the reason why Yangs are forbidden to eat heart is because many years ago, there were two close Yang cousins. They quarreled and one of the cousins murdered the other one, then cooked and ate his heart. He was punished, but as remembrance of the one who was killed, the Yang clan vowed to never eat heart again no matter where or what it is from. It did not stem from fear of curse. Rather it is their way of respecting and remembering the murdered clan member.
As far as not marrying a "Vue" or any other clan girl, that's just absurdity. Rumors and unsettled family feuds...