Did Cain marry his sister - NO definitely not.
Cain and Abel is Genesis 4
4:8 Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him.
4:14 Behold, you have driven me today away from the ground, and from your face I [Cain] shall be hidden. I shall be a fugitive and a wanderer on the earth, and whoever finds me will kill me.
4:15 Then the LORD said to him, “Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold.” And the LORD put a mark on Cain, lest any who found him should attack him.
4:17 Cain knew his wife, and she conceived and bore Enoch. When he built a city, he called the name of the city after the name of his son, Enoch.
Seth is not born until after all of this
4:25 And Adam knew his wife again, and she bore a son and called his name Seth, for she said, “God has appointed for me another offspring instead of Abel, for Cain killed him.”
4:26 To Seth also a son was born, and he called his name Enosh. At that time people began to call upon the name of the LORD.
Other sons and daughters to Adam and Eve are not mentioned until Genesis 5 and after Seth
5:4 The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters.
5:5 Thus all the days that Adam lived were 930 years, and he died.