On three separate occasions in the book of Genesis a man decides to pass his wife off as his sister during a trip out of the country because, he rather extraordinarily claims, he is likely to be murdered if people knew they were married. The first instance, between Abraham (still Abram at this point in the narrative) and his wife Sarah comes in Genesis 12:
"Now there was a famine in the land, and Abram went down to Egypt to dwell there, for the famine was severe in the land. And it came to pass, when he was close to entering Egypt, that he said to Sarai his wife, “Indeed I know that you are a woman of beautiful countenance. Therefore it will happen, when the Egyptians see you, that they will say, ‘This is his wife’; and they will kill me, but they will let you live. Please say you are my sister, that it may be well with me for your sake, and that I may live because of you.”
In Genesis 20 he tries it again:
And Abraham journeyed from there to the South, and dwelt between Kadesh and Shur, and stayed in Gerar. Now Abraham said of Sarah his wife, “She is my sister.”
And in Genesis 26 their son Isaac carries on the tradition with his wife Rebekah:
So Isaac dwelt in Gerar. And the men of the place asked about his wife. And he said, “She is my sister”; for he was afraid to say, “She is my wife,” because he thought, “lest the men of the place kill me for Rebekah, because she is beautiful to behold.” Now it came to pass, when he had been there a long time, that Abimelech king of the Philistines looked through a window, and saw, and there was Isaac, showing endearment to Rebekah his wife. Then Abimelech called Isaac and said, “Quite obviously she is your wife; so how could you say, ‘She is my sister’?”
Whether this was a more widespread practice we cannot be sure, but I like to think this was a personally idiosyncratic, superstitious habit that Abraham seemed to think would never fail him, no matter the trouble, like baseball players who refuse to change their socks when their team is on a winning streak. Everyone always sees through the ruse immediately, but he still deploys it with the confidence of Elizabeth Taylor handing some guy her earring in a White Diamonds commercial, saying, "These have always brought me luck."
If pretending your wife was your sister got you out of a jam once, it's bound to work again:
"I don't like the look of those clouds on the horizon. Might be a bad storm coming. Just to be on the safe side, Sarah, let's tell everyone you're my sister."
"How would that help?"
"Just to be on the safe side. I think it would be safer."
"But how would it make any difference, whether there was a storm or not, if I were your sister instead of your wife?"
"Well, it certainly can't hurt."
"No, I suppose it can't hurt."
Or on Isaac's departure to Gerar:
"Son, before you go, a word to the wise – when in doubt, tell people your wife is your sister."
"Why, what good would that do?"
"It's generally safer. You buy low, you sell high; you always get insurance before you need it; when you meet a lot of strangers, you should pretend your wife is your sister. Just part of an overall suite of common-sense, low-risk contingencies that I'm passing on to you, so that you might have the benefit of my years of experience."
"What if I just tell people she's my wife? Where's the harm in that?"
"It's good luck. It's tradition. It always worked for me. It's not raining, now is it? Who do you think is responsible for all the good weather we've been having?"