Back to Cain and Abel - there is nothing in scripture to indicate that Adam and Eve had no other adult children at the time Cain killed Abel. It is most likely that the inhabitants of Nod might want to kill Cain precisely because they were related and so would want vengeance for Abel’s murder.
As for Cain’s wife, it is entirely possible they.were related. We know that Abraham and Sarah were half-siblings, he admits as much Pharaoh while hiding the fact that she is also his wife. It seems to have been an issue, like polygamy, that only became unacceptable in later times.
Well, think a bit about this, and about the Catholic families you know who have children just reaching adulthood. What do you see in that “average family’s” pew at Mass? - 4, 5, or more children?
Then consider it would take several years for a person to be given his first flock of his own sheep from his father or his first plot of farmland and clearing of that land. All the while his sisters are reaching adulthood.
Now back to Cain and Abel - that is happening with them before the first offerings to the LORD. And Adam and Eve continue to have more children, I would guess girl children, or Scripture would have listed the names (boy children get more publicity there). And even after the tragedy of Cain killing his brother, Adam and Eve live long, long lives, still having more children.
Cain probably was already married, and providing for his family with the first harvest of his crops, because it only states that he brought “an offering”, not the “first fruits”. I will guess that at the time of the offering from Abel, since it was the firstborn of his sheep, he was just starting out, and was not yet married - he was giving his whole life back to God (for there is no guarantee of a second lamb after a first lamb - he is loving God by giving himself to God). I will also guess that Abel was aware of his brother’s offering, realizing that it was not right - Life is from God, and we, therefore, should give our whole life to God, since we are in the image and likeness of God. That is the sacrifice we offer at Mass today - Jesus is our life, and we are offering him to the Father, “sacrificing our only hope of life, Jesus”.
So, it is very easy to think of Cain having a wife and family when you realize that these few verses in Genesis actually lasted probably more that twenty to thirty years…