Other people living at that time could have come from the same bloodline, since 1. Eve is called the mother of all living, and 2. we do not know how many years had elapsed since everything began, and in even just a couple of hundred years, with people living to around 900 years of age, there could have been several hundreds of thousand people by then. Cain building a city in his lifetime is testament to rapid population growth. Even if he did select a woman from Nod, they could have easily been related to him also given that people would have moved away as the population grew, naturally forcing expansion into new territory in order to provide resources for the growing needs of the people. There were no laws against intermarriage among close relatives, and genetic mutations were so much not a problem that God encouraged marriage between family members (Genesis 17:9; Genesis 28:10-15). Cain could have married a niece or great niece with no genetic mutations because the human genome had not yet been corrupted, and geneticists agree that the human genome devolves or degrades as it accumulates irreversible genetic errors over time. God’s law concerning incest was not instituted until the Law of Moses came into effect..