My mother said that the bible states that it is okay for adults to have sex with their children and that is why she does not believe anything in the bible. Does the bible say this somewhere? Also I have read in the King James Version lots of stories about Men sleeping with women that they are not married to and women other than the ones that they are married to. I get confused.
Anytime somebody claims the Bible says something, you should ask them to show you where. Sometimes, like this time, they may be wrong. There is no place in the Bible that says it is acceptable for adults to have sex with their children. In fact, the Law of Moses specifically forbids it. That can be found in Leviticus 18:17. (Chapters 18 and 20 deal with laws against incest.) For my previous answer about incest, please go to What Does the Bible Say About..Incest?"
If that is the only reason your mother doesn’t believe the Bible, then pointing out that it does not say so may change her mind. I suspect, though, that this is just one excuse among many.
There are stories in the Bible about people sleeping with someone other than their spouse. In most cases you will also find that they are punished for doing so. The most famous such case is about David and Bathsheba. This can be found in the 11th and 12th chapters of 2 Samuel. David saw Bathsheba, the wife of one of his commanders, and desired her. When he found that she was pregnant with his child he tried to cover it up by having her husband called back from the war and sleep with her. When he wouldn’t sleep with his wife (out of respect to his comrades who were away from their wives) David had him killed, and married Bathsheba. In chapter 12, a prophet comes to David and tells him a story which is really about him taking another man’s wife. When David realizes what he has done he says, “I have sinned against the LORD.” As a result, the child dies.
Throughout the Bible the sin of adultery (sleeping with someone else’s spouse) is condemned. In fact, God is so against it that he has some of the prophets tell the people that their going away from God to idols is the same thing as adultery. (This can be found especially in the books of Jeremiah and Ezekiel.)
While the Bible does tell about certain cases of adultery, it does not say that they were right in doing it. It just shows that people are people, with all their faults. That does not mean we should follow the example of their faults.