Good question!
Let's ask Joseph Smith Jr, the most honored prophet of mormonism --
In Pearl of Great Price, Book of Abraham, Chapter 2:22-25, he relates the following --
22 And it came to pass when I was come near to enter into Egypt, the Lord said unto me: Behold, Sarai, thy wife, is a very fair woman to look upon;
23 Therefore it shall come to pass, when the Egyptians shall see her, they will say—She is his wife; and they will kill you, but they will save her alive; therefore see that ye do on this wise:
24 Let her say unto the Egyptians, she is thy sister, and thy soul shall live.
25 And it came to pass that I, Abraham, told Sarai, my wife, all that the Lord had said unto me—Therefore say unto them, I pray thee, thou art my sister, that it may be well with me for thy sake, and my soul shall live because of thee.
So according to Joseph Smith, God commands lying and tempting others into lying.
But wait!
In the Book of Mormon, authored by Joseph Smith Jr (oops, I mean translated by Joseph Smith Jr, I mean authored by ... aw, never mind), we read that God takes a dim view of lying --
Ether 3:12
12 And he answered: Yea, Lord, I know that thou speakest the truth, for thou art a God of truth, and canst not lie.
2 Nephi 9:34
34 Wo unto the liar, for he shall be thrust down to hell
Your premise is flawed, Bonaparte. Sarai was Abraham’s sister. They shared a parent.