Fu Xi
In some variations of the flood myth, Fu Xi and Nüwa were these two siblings. It went like this: A disastrous flood erased all of humanity except for Nüwa and her younger brother, Fu Xi, who both happened be on a boat at just the right time. After the floodwaters receded, they found out that they were the only people that remained. As the sole survivors, it was their duty to not allow their race to go into extinction. It was their responsibility to continue the survival of mankind and there was only one known way. They realized that they would have to procreate, but they were very uncomfortable with the idea since they were siblings. Fu Xi and Nüwa decided to look for nature's guidance so they came to the decision that they had to go through a trial that could prove whether they were fated to be man and wife. Fu Xi and Nüwa each climbed a different mountain that faced each other. They each lit a fire on their respective mountain. If the smoke blew straight up, they would take it as sign that they should not get married. However, to their great astonishment, the smoke trails from their fires intertwined with one another. It was a definite sign that they should get married. So Fu Xi and Nüwa became man and wife and through their union, the earth was repopulated.