All the current seer and knight players are related. Kankri/Karkat and Laluta/Terezi are dancestors while Dave and Rose are siblings.
It is possible the Seer and Knight classes are meant to be related, no matter what.
http://www.mspaintadventures.com/?s=6&p=007302
http://www.mspaintadventures.com/?s=6&p=007303
http://mspaintadventures.wikia.com/wiki/Terezi