Abraham is traditionally known for being the father of many nations. He is also known for his descendants Isaac and Ishmael birthing two major world religions: Judaism and Islam. Abraham is also known for having "great faith" in God. He is known in this manner because he trusted that God would give him a son from Sarah (even after he had a son with Hagar) and because he trusted in God when he was sacrificing Isaac.
It is common for people to criticize Abraham for his near sacrifice of Isaac. "What kind of father would sacrifice his son?" some ask. Some take it as a criticism of Abraham and others take it as praise.
Nevertheless, though Sarah is praised for his extraordinary beauty, it is almost always forgotten that Abraham married his half-sister. Perhaps this part of the story is passed over because the truth falls in an obscure story that is also normally passed over.
The story in which the reader learns that Sarah is Abraham's half-sister is in Genesis 20. This story perhaps is passed over because it paints Abraham as a con artist. Abraham lived in the region of Negev and while he was there, Abraham did not admit that Sarah was his wife. He merely said, "She is my sister" (Genesis 20:2). Abimelech, the king of Gerar, "took" Sarah.
God, however, came to Abimelech in a dream and said that he is as good as dead because Sarah is a married woman. Abimelech is taken aback because he was duped. Sarah even said that Abraham is her brother. He feared that his nation will be destroyed. Abimelech asked Abraham why he did this and Abraham said that he did it because there was no fear of God there. He apparently wanted to instill a fear of God in that place.
Then, Abraham admits that Sarah is his sister and wife. "She really is my sister, the daughter of my father, though not of my mother; and she became my wife" (Genesis 20:12). Abimelech then brought cattle and money to Abraham to appease him and God. After Abimelech had innocent motives and asked for forgiveness for what he unknowingly did, God "closed up every womb in Abimelech's household" (Genesis 20:17).
Sarah was clearly Abraham's half-sister. The reason why this fact is over passed over is unclear besides the fact that people desire to paint Abraham in the best light as possible.