I have a question: Were there any other people on Earth at the time of Cain and Abel or at the time of Adam and Eve were created? Because the Bible says Adam and Eve were the first two humans created. However, in Genesis, after Cain killed Abel, he was afraid of other people finding him, and God gave him a mark which admit that there were other people besides the four (Adam, Eve, Cain, and Abel). Also, Cain went to the land of Nod and married a woman out of nowhere.
My cousin was arguing that God also created other people besides Adam and Eve. But I explained that the Bible specifically states that God only created Adam and Eve as the first two humans. Cain probably married his sister or niece or grandniece since the Bible doesn't specify his age and where his wife came from. Cain was afraid of his other brothers and sisters. So even though only Cain and Abel were mentioned at the time, there were also other brothers and sisters of them descended from Adam and Eve because they both lived really long and had numerous children, although the Bible mostly only mentioned the men and their sons.
"And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth. After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters" (Genesis 5:3-4). Seth was born after Abel was killed (Genesis 4:25).
We don't know exactly how long it was before Adam and Eve had Cain and Abel. Let's assume that Cain was born the first year after Creation, then 128 years have gone by when Cain killed Abel. We don't know how many children Adam and Eve had or how far apart they were. And in 128 years those children could have married and had children themselves.
You are correct that Cain's wife would have been one of his sisters or one of his nieces. That was allowed at the beginning because there was no genetic defects then. After the flood, it was unusual to marry close siblings, which is why Pharaoh was surprised that Abram married his half-sister, Sara (Genesis 12:18-19). By the time of Moses, marrying close siblings was forbidden (Leviticus 18:6-18). Today, marrying even cousins is frowned on.