H
Could Cain have married someone he was related ? Yes that is possible but does the scriptures say that Cain married someone related to him ? The scriptures does not say either way, so one therefore can only assume, as one can not prove anything one way or another. Adam had sons and daughters after Seth, not before Seth, Seth was a replacement for the loss of Abel, therefore Cain already killed Abel before Seth was born, Cain went east and took a wife, Seth was born and after Seth, Adam had many sons and daughters. The odds of Cain taking a sister as a wife, would be zero to none.
That was plenty for me to see that you are scripturally stunted,
and therefore not worth wasting any more of my time on.
G'day.