Dictionary:
adultery (ə-dŭl'tə-rē, -trē)
n., pl. -ies.
Voluntary sexual intercourse between a married person and a partner other than the lawful spouse.
Your reading comprehension and/or understanding of certain words is lacking.
Though the book of Genesis condemns sexual relations between children and their parents, it nowhere prohibits a man from marrying his sister or niece. Abraham, for example, married his half-sister without compunction. Not until the time of Moses were laws established forbidding a man from marrying a sister or niece. The timing of this command makes perfect sense biologically, for genetic defects as a result of intra-family marriage would not begin to crop up until after the first few dozen generations.
As far as Adam's children populating the earth, I don't expect you to understand everything about the Bible, but you certainly don't hesitate to mock what you don't understand. If you really do want to learn something, and aren't just being contentious, then I recommend
this article and
this article. Otherwise, I'm done talking to you here, as I've offered plenty of evidence for you to consider, and you are being completely closed-minded.
As far as proving God to you, I don't have to do that and I'm not interested in arguing whether or not He exists; He does, and you will meet him one day. And you'll give an account of your life to Him. And if you haven't repented of your sins by then, you'll be under His eternal judgment. That's your problem, not mine.
Whosoever shall fall upon that stone shall be broken; but on whomsoever it shall fall, it will grind him to powder.
Luke 20:18