So I'm guessing Yorichi spent his remaining years searching for his brother after he became a demon, which would explain why when Tanjiro's ancestor mentioned about Yorichi settling down he rejected that idea. You could say Yorichi's will to find his brother who had become a demon, kept him alive to old age considering those with the mark dies early. Also the demon brother didn't understand Yorichi even to and after Yorichi's death, he never stopped and tried to understand at all. He just kept wallowing in his own jealousy towards Yorichi and his abilities, but it's also sad too because he never stopped to appreciate what he had or the brother who loved him.
Now we'll finally get to know how Muzan survived his encounter with Yuichi, when it was stated that Yuichi drove him to the brink of death, which would explain why Muzan is hellbent on killing everyone related to the sun breath user and also fears him.