By what ruse did Abram fare well in Egypt?

Abraham told the Pharaoh that his wife Sarah was his sister, for fear that the Pharaoh would kill him in order to have sex with her (Genesis 12:13), only to be found out and banished from Egypt. Later, for the same reason, he told Abimelech, king of Gerar, that his by now quite elderly wife (Sarah was now over 90 years old and stricken with age) really was his sister.

Just as Abraham tried to pass Sarah off as his sister while visiting Abimelech in Gerar, so did Isaac, his son, try to pass his wife Rebekah off as his sister to the same king, once again for fear that the king would kill him in order to have sex with her. It should be questioned why Abraham and Isaac believed that Egyptians and Gerarites would only attempt to have sex with a woman after killing her husband, and why they would not kill her brother in order to achieve the same ends, but the Book of Genesis describes this as a successful ruse, at least for a while.