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.
This was the case. See Genesis 11:29
Genesis 20:8-12
8 Therefore Abimelech rose early in the morning, and called all his servants, and told all these things in their ears: and the men were sore afraid.
9 Then Abimelech called Abraham, and said unto him, What hast thou done unto us? and what have I offended thee, that thou hast brought on me and on my kingdom a great sin? thou hast done deeds unto me that ought not to be done.
10 And Abimelech said unto Abraham, What sawest thou, that thou hast done this thing?
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.