It doesn't really say but in Chapter 11: verse 27 through 30: Now these are the generations of Terah: Terah begat Abram, Nahor, Haran; and Haran begat Lot. And Haran died before his father Terah in the land of his nativity, in Ur of the Chaldees. And Abram (Abraham) 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. 30 But Sarai was barren; she had no child. The name Nahor means hoarse dry hot and could refer to a desert place where it is because The Origin of The Name Nahor is from Israel and Israel had Two deserts: Negev and Yehuda. So if that being The Case, Verse 30 says that Sarai was barren. The word Barren means not producing or incapable of producing offspring; unproductive; unfruitful: barren land. and also can refer to a Desert that is Barren. If that is True then Nahor's wife, Milcah could have somehow have been Sarai's Mother. The Bible says in The Book of Genesis Chapter 11 verse 27: Now these are the generations of Terah: Terah begat Abram, Nahor, and Haran; and Harans begat Lot. And Haran died before his father Terah in the land of his nativity, in Ur of the Chaldees. If Haran had a Child first then maybe Nahor who was Haran's Brother and his wife Milcah had a Produced Sarai who became Abrahams wife next after his death for Abraham to have a wife. The Name Milcah means Queen and Sarai means "my princess" !!! Sarai could have been Milcah's Daughter, A princess is a nonreigning female member of a royal family and also (in Great Britain) a daughter or granddaughter (if the child of a son) of a king or queen. Also down The Royal lineage of Shem would The Messiah come who is Th Lord Jesus Christ. From The loins from Abraham and Sarah would come the Jewish people, raised up for the express purpose of giving the world the Word of God and, as well, serving, one might say, as the womb of the Messiah. According to Chapter 17:15 And God said unto Abraham, As for Sarai your wife, you shall not call her name Sarai ("my princess," referring to the fact that she was Abraham's princess alone), but Sarah shall her name be (simply means "princess;" the idea is, whereas she was formerly Abraham's princess only, she will now be recognized as a princess generally and, in fact, in a sense, could be referred to as the "Mother of the Church")

Sarah was Abraham's half-sister.

Gen 20:11-12 MKJV And Abraham said [to Abimilek]... (v.12) And yet truly she is my sister. She is the daughter of my father, but not the daughter of my mother. And she became my wife.

The name of Sarah's mother is not given in scripture, but Adam Clarke in his Commentary on this verse in Genesis says that Ebn Batrick, in his annals says her name was "Tehevita":-

Gen 20:12 -
She is my sister - I have not told a lie; I have suppressed only a part of the truth. In this place it may be proper to ask, What is a lie? It is any action done or word spoken, whether true or false in itself, which the doer or speaker wishes the observer or hearer to take in a contrary sense to that which he knows to be true. It is, in a word, any action done or speech delivered with the intention to deceive, though both may be absolutely true and right in themselves. ...
[Sarai is] [T]he daughter of my father, but not - of my mother - Ebn Batrick, in his annals, among other ancient traditions has preserved the following: “Terah first married Yona, by whom he had Abraham; afterwards he married Tehevita, by whom he had Sarah.” Thus she was the sister of Abraham, being the daughter of the same father by a different mother.

(However, her mother's name cannot be verified in Scripture.)