Did adam and eve actually exist?

adam and eve according to the bible had two sons cain and abel. so where did the rest of us come from? the way i see it is that adam and eve would have had to have had a daughter in which case one of the sons would have had to have slept with her or the sons would have had to have slept with the mother to produce others. either way the bottom line is that we all must have decended from a ual relationship which surely the bible would consider wrong