Where did Cain's wife come from? Who were the other people he was saying would kill him?
God first created Adam and from Adam he created Eve. Adam called his wife's name Eve because she was the mother to be of all living. This means that Cain must have married one of his sisters. Incest was not a danger to health at the beginning - there was no alternative.
The people that Cain was afraid would kill him would be other children of Adam and Eve who would be born and would hold him guilty for killing Abel.
Bear in mind that they lived very long lives and produced other children besides their next named son Seth.
Genesis 5:3-7 And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth. 4 After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters. 5 So all the days that Adam lived were nine hundred and thirty years; and he died. 6 Seth lived one hundred and five years, and begot Enosh. 7 After he begot Enosh, Seth lived eight hundred and seven years, and had sons and daughters.
I hope this helps.
God bless,
Glenn