Answer:
Some might say they are, depending on interpretation. In ancient times, there was a priest Seto, who Seto Kaiba is an apparant reincarnation of. Priest Seto was the secret son of priest Akhenaden, brother to the previous pharaoh, and Yami's uncle. So priest Seto was Yami's cousin.
The main issue is though, is Seto Kaiba actually a blood descendant of priest Seto, or an unrelated (non-family related) reincarnation of him. If he is the former, then technically he is related to Yami. If the latter, then he's probably not.