Liam,
As was pointed out previously, the information expressed in Genesis has a flow that unites adam1 to Adam 2 making them the same person. In the Gen 5 quote below, God unites the supposed two adams and shows that adam 1 is Adam 2.
Genesis 5: This is the book of the generations of Adam.
In the day that God created man, in the likeness of God made he him; 2 Male and female created he them; (adam 1) and blessed them, and called their name Adam, in the day when they were created.
3 And Adam (Adam 2) lived an hundred and thirty years,". So adam 1 and Adam 2 is this Adam that lived a hundred and thirty years and so on…
The initial declarations of God about the creation of mankind dictate the flow of what comes after the initial creation.
Since mankind was created in God’s image and without sin I conclude that it would have been proper for sinless humanity to become one flesh with their siblings. But as Jesus said in Mark 10:6 "But at the beginning of creation God ‘made them male and female.’ 7 ‘For this reason a man will leave his father and mother and be united to his wife, 8 and the two will become one flesh.’ So they are no longer two, but one. 9 Therefore what God has joined together, let man not separate." So marriage between parent and offspring would not be in God’s design.
As far as Cain building a city, it appears that the word can mean, a city, a place guarded by waking or a watch, a mere encampment or post. So Cain went to an area called Nod east of Eden and started to build a encampment-a guarded area-a city. This statement does not demand an existing population of people already there who needed a metropolis to live in, he simply began to build encampment or settlement.
In reference to scripture not mentioning that his wife was or was not his sister is not necessary, as the already declared statements of God clarify that.
At the beginning of creation God made one man and then from him he made woman, from them all mankind is produced.