TM7, I will be as brief as possible but, your analogy is OUT OF SCRIPTURAL CONTEXT.
Genesis Chapter 11 verse 31. And Terah took Abram (Abraham) his son, and Lot the son of Haran his son's son, and Sarai (Sarah) "HIS DAUGHTER IN LAW" , his son Abram's (Abraham's) wife; and they went forth with them from Ur of the Chaldees, to go into the land of Canaan; and they came unto Haran, and dwelt there.
In this scripture Sarah is CLEARLY DESCRIBED, as Terah's DAUGHTER IN LAW, and there is NO MENTION of her being his daughter. Mosiac Law FORBID the marriage between brother and sister, and Abraham was not only the first Hebrew but, was a strick adherent to that Law.
Here is your out of contextual confusion as to them being brother and sister. When they were travelling into Egypt, they were both afraid that, because of Sarah's beauty, Pharaoh would have Abraham killed and take Sarah as his wife, so they made a pac to claim they were actually brother and sister to save Abraham's life, if it became necessary, AND IT DID BECOME NECESSARY.
This pack is described and the reasons for it in Genesis Chapter 11 verses 11 thru 13.
Their fears were realized when Pharaoh saw her beauty and made the move to make her is own wife Genesis Chapter 11 verses14 thru 16. With this done, God began to PLAGUE THE PHAROAH, for taking Abraham's wife, and Pharaoh FOUND OUT THE TRUTH, and confronted Abraham, and gave Sarah back to him, and sent them on their way. Genesis Chapter 11 verses 17 thru 20.
As far as Sarah giving Abraham her handmaid "Hagar" in marriage, she had no authority to do so under MOSAIC LAW, and the union was ILLEGITIMATE, from the get go, and the product of that union "ISHMAEL" was not deemed Abraham's RIGHTFUL FIRST BORN, and later God fulfilled his promise, and Sarah gave birth to Issac the rightful heir.
Ishmael was blessed for the simple reason that he was Abraham's son, but could not inherit the kingdom as God had promised his first born would.
Abraham was not Sarah's sister, and Ishmael WAS born out of wedlock. You'll have to take up your argument with God, and you will surly lose if you do.