1) It isn't incest unless a) it's against the Law and b) you're the only damn people on earth.
2) That is NOT what happened though you need to read better. Genesis 5 :4 says they had many other sons and daughters.
3) Cain could have been 300 years old when he killed Abel.He may have had sisters and brothers and nieces and such that were 250 years old had propigated and spread so far ,they hardly knew each other.
4)Cain could have married a niece or a sister.
5) Incest was common even ,Abraham married sarah ,she was his 1/2 sister.
6) Not until the time of Moses did God say "Ok the gene pool is polluted and weakened enough,no more."
Remember ,the first humans were perfect and as the generations passed ,after the fall of Adam,the ages got shorter and shorter,until after Noah it went from 900 years to 120 years.By the time of David it was 70 years average.
Try learning before you try to put things down.