Adam Clarke on Genesis 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. See note on Gen 12:13.

The 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.