Cain married one of Adam and Eve’s other daughters. It would be his sister technically, but it would not be the same is if you or I married our sister...This was the beginning of the human race and Adam and Eve would have been quite prolific.
... and your evidence for this is what, exactly?
It would be his sister technically
"technically" ? ? ?
Why qualify it? ANY female child begotten of MY mother and MY father is MY sister, no "technical" qualifications needed.
I here you, and this is the usual answer,
but incest is prohibited in Leviticus 18
Specifically between siblings.
I wish Genesis was clearer on this...