Question: "Who was Cain's wife? Was Cain's wife his sister?"
Answer: The Bible does not specifically say who Cain’s wife was. The only possible answer was that Cain's wife was his sister or niece or great-niece, etc. The Bible does not say how old Cain was when he killed Abel (Genesis 4:8). Since they were both farmers, they were likely both full-grown adults, possibly with families of their own. Adam and Eve had surely had more children than just Cain and Abel at the time Abel was killed - they definitely had many more children later (Genesis 5:4). The fact that Cain was scared for his own life after he killed Abel (Genesis 4:14) indicates that there were likely many other children and perhaps even grandchildren or great-grandchildren of Adam and Eve at that time. Cain's wife (Genesis 4:17) was a daughter or granddaughter of Adam and Eve.
Since Adam and Eve were the first (and only) human beings, their children would have no other choice than to intermarry. God did not forbid inter-family marriage until much later when there was enough people that intermarriage was not necessary (Leviticus 18:6-18). The reason that incest often results in genetic abnormalities in children is that when two people of similar genetics (i.e. a brother and sister) have children – genetic defects are far more likely to result because both parents had the same defects themselves. When people from different families have children – it is highly unlikely that both parents will have the same genetic defects. The human genetic code has become increasingly “polluted” over the centuries as genetic defects are multiplied, amplified, and passed down from generation to generation. Adam and Eve did not have any genetic defects, so that enabled them and the first few generations of their descendants to have a far greater quality of health than we do now. Adam and Eve’s children had few, if any, genetic defects. As a result, it was safe for them to intermarry. It may seem strange or even disgusting to think of Cain's wife being his sister. In the beginning, since God started with one man and one woman, the second generation would have no choice but to intermarry amongst themselves.