Yes i agree with Simonline.
Sarah was the daughter of Haran. Haran was the elder brother of Abraham i.e Haran and Abraham were the sons of Terah. According to the Book of Jasher (mentioned in the OT-Bible in Joshua 10:13 and 2 Samuel 1:18) Haran was 32 years older than Abraham. Abraham was 10 years old when Sarah was born to Haran. So, Abraham was not lying. In a way Sarah was a close relative of his (i.e niece) and more like a sister to him.
Genesis 20:12 - 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.
Here, Abraham says that Sarah is the daughter of his father Terah to imply that she was born to Terah's eldest son Haran and that Sarah belonged to the genealogy of his father and not his mother.
There is a book called Book of Abraham which did not make it to the Bible Canon but provides extra information regarded Abraham and his life similar to the Book of Jasher. I hope the following chapter from the Book of Abraham would make a good read as part of this topic.
CHAPTER 50
NOW, among the believers were Lot, the son of Haran, and Haran's daughters, Milcah and Sarai.
2. Milcah was fair, but Sarai was beautiful above all women.
3. Her face was as that of an angel filled with light, her cheeks as two roses in full bloom, her hair as spun gold which men treasure above all, her eyes as pools of blue reflecting the glory of God's sky, her nose delicate and lovely, and her countenance truly was fair as the sun.
4. Her breasts like two mountains rising above the plain of Shinar were fair to behold, and her complexion truly like the clouds in purity.
5. Her arms were comely, her hands perfect and delightful to behold, always in the service of those in need and quick to hasten to the service of the Lord her God.
6. Her palms were lovely, unmarred by her constant labor, her fingers long and slender.
7. Her feet were comely and always set in the path of truth.
8. Her thighs were well­rounded and soft, yet never failing to sustain her in her labor of love in behalf of the servants of God.
9. Truly no maiden was fairer than Sarai, the daughter of Haran, for her beauty was greater than all other women and she excelled them all.
10. But with her beauty was great wisdom and perfect faith in the Lord and constant service in behalf of the saints, for her hands never cease from blessing the people of God.
11. Wherefore, my brother Nahor took Milcah, the daughter of Haran, to wife, but I Abram, took Sarai at the direction of the Lord and I found great joy in her.