What was Avraham trying to achieve by claiming that Sarah was indeed his sister?
Rashi: Avraham was pointing out to Avimelech that he did not lie outright, in that his claim was partially true.
Ramban, Seforno: He was incriminating Avimelech, by blaming him for taking Sarah without enquiring as to whether she was also his wife too.
How could Avraham claim that Sarah was indeed his sister in order to substantiate his initial claim? Why would that not be incest?
Rashi and Rashbam: Because, asuming the Avos to have been considered B'nei No'ach, a paternal relative of a ben No'ach is not considered a blood-relative (and Haran was Avraham's brother from another wife of Terach).
But she wasn't his sister; she was his niece (his brother's daughter)?
Rashi and Rashbam: It was perfectly in order to claim that she was his sister, due to the principle 'grandchildren are like children (in which case Haran's daughter was akin to Terach's daughter).
What point was Avraham making? Seeing as he had caused Avimelech to sin, what difference did it make whether his claim that Sarah was his sister was true or not?
Ramban: Countering Avimelech's question why he tricked him, based on the fact that he (Avimelech) did not make a habit of taking people's wives, Avraham answered that, not knowing the ways of Avimelech, he was merely following a pact that he had made with Sarah, as he goes on to explain - not because he had seen anything that would implicate them, but because most countries through which he passed lacked Yir'as Hashem.