(We may have discussed this before, but I can’t find it anywhere).
What’s the deal with Abraham claiming that Sarah was his sister while they were in Egypt? Does it have anything to do with the (purportedly) widespread practice of incest in ancient Egypt?
He was afraid that if he presented himself as her husband, an Egyptian would kill him in order to marry Sarah, whereas if he acted as her brother, an Egyptian might still hope to marry her but would attempt to do so through Abraham’s permission (which of course he’d deny), as brothers would sometimes act as “guardians” of their younger sisters (in the absence of the girl’s father).
Darmok and Jalad at Tanagra!
Sorry, it’s probably just me
Sokath, his eyes uncovered
Yeah, its just you! 
Sarai actually was Abram’s half-sister, wasn’t she? At least, I think I remember Abram claiming that after getting caught. I also remember he tried the “she’s not my wife, she’s my sister” trick at least twice, and Issac did it once more, each time nearly bringing plagues down on the would-be paramours.
Shaka, when the walls fell.
Sarai was certainly a kinswoman of Abraham’s, but not necessarily his biological sister.
Jewish tradition identifies Sarai as Yischah (Jessica), mentioned at the end of Genesis 11. That would make her his neice – Yischah being the daughter of his brother Haran.
(As a further side note, Jewish tradition maintains that while the Patriarchs weren’t obligated to keep all the commandments [since they weren’t given yet], he would have to keep the Noachide commandments. One of those commandments is against sleeping with one’s sister.)
The Bible often uses the word “brother” for “kinsman” in general. For example, Laban calls Jacob his “brother,” when he, in fact, was his uncle.
Zev Steinhardt
Which is prety meaningless ince he had already lied and he was afraid the man was about to kill him for lying.
What would you do if some big guy with a sword confronted you like this? Say “Yeah, I made the whole thing up to make you look like a fool” or “I didn’t really lie, she is my half-sister?”
zev:
Actually, Zev, I think the Noahide prohibition is only on sleeping with one’s full sister, but a half-sister is not considered incest by Noahide law.
Blake:
I don’t think he was afraid that Pharaoh was going to kill him at that point, because that was after G-d struch Pharaoh with blemishes to show him that Abram and Sarai were special to him. He probably just felt that a) it would be a good idea to show respect to Pharaoh as a matter of protocol, so therefore explain that he hadn’t intended any genuine falsehood, and/or b) his own personal sense of honesty would have been insulted if he were thought to be a genuine liar, so he explained how his original statement was, in a sense, truthful.
According to the Rambam, the prohibition is against sleeping with your maternal sister…a sister who has the same mother. And, in proof of this, he uses Abraham, actually. 
From the Laws of Kings and Their Wars:
Hmm… maybe I’m just a perv and was reading more into the passage than was meant. I’d sorta thought that maybe the implication between the lines was that it was obvious that Abraham and Sarah were an couple. So to hide the fact they were actually husband and wife, Abraham claimed Sarah was his sister/mistress (acceptable by incestuous Egyptian standards), meaning that she was still technically available for marriage.