The short answer is no, Abraham did not really marry his sister. I think the explanation for this is interesting. I hope you will find it enlightening.
Here is the quote from Genesis 11:29.
And Abram and Nahor took them wives: the name of Abram's wife was Sarai; and the name of Nahor's wife, Milcah, the daughter of Haran, the father of Milcah, and the father of Iscah.
Let's take a look at two verses dealing with Abram and Lot.
And they took Lot, Abram's brother's son, who dwelt in Sodom, and his goods, and departed.
And he brought back all the goods, and also brought again his brother Lot, and his goods, and the women also, and the people.
Did you notice that in Genesis 14:12 Lot is called Abram's brother's son or his nephew, as we would say today, and then in Genesis 14:16 Lot is called Abram's brother. So one might suggest that in Abram's day the word brother could be understood to also include nephews. Let me suggest that the same is true for the word sister.
I have learned that in the Hebrew language a noun can be singular or plural based on the context. Strong's says that the Hebrew word for daughter is bath, 1323. If you do an Englishman's search for this word you will see that quite a number of the instances are translated as daughters and also some are daughter. So now here is Genesis 11:29 with the word daughters instead of the singular daughter.
And Abram and Nahor took them wives: the name of Abram's wife was Sarai; and the name of Nahor's wife, Milcah, the daughters of Haran, the father of Milcah, and the father of Iscah.
Interesting! So now let's get a second source for this idea. The Book of Jasher, while not inspired, is mentioned twice in scripture that we should look at it. So here is what it says.
Jasher 9:14 And Haran, the son of Terah, Abram's oldest brother, took a wife in those days. Haran was thirty-nine years old when he took her; and the wife of Haran conceived and bare a son, and he called his name Lot. And she conceived again and bare a daughter, and she called her name Milca; and she again conceived and bare a daughter, and she called her name Sarai. Haran was forty-two years old when he begat Sarai, which was in the tenth year of the life of Abram; and in those days Abram and his mother and nurse went out from the cave, as the king and his subjects had forgotten the affair of Abram.
And at that time Nahor and Abram took unto themselves wives, the daughters of their brother Haran; the wife of Nahor was Milca and the name of Abram's wife was Sarai. And Sarai, wife of Abram, was barren; she had no offspring in those days.
Haran had just died before they took his daughters as wives. Abram then also took upon himself the care of his nephew Lot. Also according to Jasher Abram spent a number of years living in the house of Shem being taught the law of God. So Abraham knew God's law very well and would have known it was improper for him to marry his sister. Sarah was actually Abraham's niece. Leviticus 18 does not prohibit the marriage of a man and his niece. So it is helpful to remember how Abraham understood the word sister when reading the following.
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.
In Genesis 26 Isaac did the same thing as his father. Abraham and Isaac both deceived the king in order to protect themselves from being killed. I think we can safely suggest that they knew that this did happen. Isaac said that Rebekah was his sister. Scripture tells us though that Rebekah was the grand daughter of Abraham's brother Nahor and his wife Milcah who was Haran's daughter. Isaac could say that Rebekah was the daughter of his father, but not the daughter of his mother. There was no word for grand father. So he could be a bit more clear and say that Rebekah was the daughter of his father's father's other son and further explain that she was the daughter of that other son Nahor through Buthuel the son of Milcah. It gets complicated. Today we might call her a first cousin, once removed. They were double cousins as well. Just remember that the word brother and sister can include more in Hebrew than it does in English.