you will find your answer in Gen. 5:4 "and he begat sons & daughters" Adam & Eve had other children besides Cain Able & Seth. Therefore Cain married his sister, this is before the genepool was poluted & it wasn't until The law of Moses came to be that is was forbidden to wed your near relative. For a further study you could try contacting www.answersingenesis.org they even have a booklit that goes into detail about it. Best wishes, Charcloth