“Adam and Eve had other children but surely if one was a daughter, she could not (or should not) have been Cain’s wife.”
Why not?
When dna of the Adam line first came to be, there were no harmful mutations to multiply. Rules came when they were needed.
Having children by a blood relative works differently for the first family than it does for later entries in the gene pool.
There are certain physiological problems, genetically speaking, with that.