The 1st sun breath user is Yorichii and the other one who look like Tanjiro is Tanjiro's ancestor. Yorichi later learn him abt the sun breathing techniques and dance and give him his earings to pass it to his descendants so that they can make his legacy still live on. So years after years the legacy was transmit until Tanjiro's dad is born and then he pass it to Tanjiro. Which make Tanjiro the last Sun Breath User
For those who's still not get it Tanjiro is not blood related with Yorichi and his twin brother but he's the successor of Yorichii.
Mushiro is the one in the story who is blood related to the twin i.e he's most likely Kokuzhibo's descendant through his son's lineage
Mushiro is the one in the story who is blood related to the twin i.e he's most likely Kokuzhibo's descendant through his son's lineage