What superstition about the gods and happiness does Bright Morning forget?

In the opening chapter of Sing Down the Moon, Bright Morning heads outside early in the morning to taste the first day of spring. As she stands alone in the peach orchard, she is overwhelmed by the new life that has sprung up out of nowhere. The previously bare trees have started to bud, and where there was once only sand, there is now the insistent gush of blue rushing water.
Bright Morning is truly enraptured by the first day of spring. She feels like leaping and dancing and singing for joy. But then she remembers the old Navajo superstition: it is bad luck to show too much happiness. And the gods will punish anyone who fails to remember this. Bright Morning believes that this is what happened to her brother. He had been incredibly happy that fateful day when he returned from a day's hunt having successfully bagged a large deer. But on his way home, he was struck by lightning and killed. Ever since that terrible day, Bright Morning has been careful not to show too much happiness, lest she suffer the same fate as her late brother.