Actually this is a pretty common question. I think we first need to remember that the Book of Genesis is not intended to give us every answer to our questions about many details of the creation of the world and man. Thus, it isn’t too surprising that this question isn't answered directly here. (Neither are we told where Satan came from, when he appears in Genesis 3.)
The generally accepted answer is that Adam and Eve had “other sons and daughters” (Genesis 5:4). It should also be recalled that people in those days lived a very long time. Adam, for example, lived to be 930 years old, so he had many opportunities to be a father, grandfather, great grandfather, etc. I think it would generally be accepted that there was not, at that point in time, a genetic problem with one marrying a sibling. Remember that Abraham married his half-sister (Genesis 20:12).