That is a good question, is Jesus Christ the same yesterday, and to day, and for ever.
For example?
Actually that is the real issue. But she was his sister.
Judge not according to the appearance, but judge righteous judgment. John 7:24
The LORD told Abimelech to restore to Abraham his wife; for he is a prophet.
That's an interesting connection, If Abraham's marriage to Sarah was sinful, why does God repeatedly reaffirm His covenant with Abraham while Sarah remains his wife?
You're right that Hebrews says: "Jesus Christ the same yesterday, and to day, and for ever." (Heb. 13:8)
But does that mean every command given under Moses was already binding in exactly the same way before Sinai?
For example, where do we find a prohibition against marrying a half-sister before Leviticus 18?
As for examples, Abraham and Sarah would be one if we understand Genesis 20:12 literally.
The sons of Adam and Eve would also have had to marry close relatives for mankind to multiply.
The question that keeps coming back to me is: What does the text itself say God thought about Abraham's marriage?
In Genesis 20, God tells Abimelech: "restore the man his wife; for he is a prophet" (Gen. 20:7).
Notice God doesn't say: "Return this unlawful woman."
He calls Sarah Abraham's wife and commands her restoration.
So I think the real question is not whether Sarah was Abraham's sister.
Genesis 20:12 plainly says she was his half-sister.
The question is: Does Scripture ever condemn Abraham's marriage on that basis?
That's the specific text I'd be looking for.
Jesus often brought people back to what is actually written rather than what we assume must follow from it.
"What is written? How readest thou?" seems like the right question here.