The Bible tells us that Abraham married his sister:
Genesis 20:11-12: 11 And Abraham said, Because I thought, Surely the fear of God is not in this place; and they will slay me for my wife's sake. 12 And yet indeed she is my sister; she is the daughter of my father, but not the daughter of my mother; and she became my wife.
However we also see the Bible later on forbids such an action:
Deuteronomy 27:22: 22 Cursed be he that lieth with his sister, the daughter of his father, or the daughter of his mother. And all the people shall say, Amen
So now we see that it's something bad to marry your sister, and you will be cursed if you do so. So hence the question, can you marry your sister or not? Should I be allowed to follow Abraham's example, God's friend as the Bible calls him, or should I go on and follow what Moses taught which is that it's forbidden and you become cursed. Which one?
The problem does not end there, as we can see, it seems God is changing his mind over time in the OT. At first it seems he has no problem with people marrying their sister, since he didn't object to Abraham. Later on with Moses he says the person who practices such an act is cursed. This causes further problems; the reason being is that God says he does not change his mind:
"God is not a man, that he should lie, nor a son of man, that he should change his mind. Does he speak and then not act? Does he promise and not fulfill?" Numbers
"He who is the Glory of Israel does not lie or change his mind; for he is not a man, that he should change his mind." 1 Samuel 15:29
So note God in the OT says he doesn't change his mind, yet we see him exactly doing that with the issue of marrying your sister. At first he didn't have a problem with it, because as we all know Abraham was not cursed, but he was greatly blessed. Later on God changes his mind, and says you will be cursed if you marry your sister. So hence he changed his mind, an obvious change of mind when he said he wouldn't change his mind!
So in summary we have quite a few problems here:
1- Abraham married his sister, and was not cursed, instead he was greatly blessed throughout his life, and it was not something wrong to marry your sister in the time of Abraham.
2- Later on God says if you marry your sister you cursed.
3- So can you marry your sister or not? Should I follow Abraham's example, God's friend, who was blessed, or should I follow Moses who says your cursed if you such a thing.
4- God said he doesn't change his mind.
5- God did change his mind on the issue of marrying your sister.
The obvious inconsistency and contradictions are all there for everyone to see. The incoherency in the Bible is obvious for all to see.
And Allah Almighty is far above such inconsistencies!