Some say that blood is thicker than water. It's also scarier than water for a lot of people. In the story of Isaac and Rebekah and their sons, Jacob and Esau, we find that Judeo-Christian ethics emerge from a culture that is very foreign and very familiar at the same time. It is the very insular and fearful culture of one family that forms the basis for biblical spirituality. This, again, refers to passages in Genesis chapters 24 through 34.
Fear is a natural human emotion, and certain circumstances and relationships can foster fear more than others. In Isaac's case, a certain amount of fear is learned from his father, Abraham, and he responds to it as Abraham did. When Isaac is living in Abimilech's land, his fear of the other men there leads him to tell the same lie Abraham had told about his wife on more than one occasion. Isaac identifies Rebekah as his sister. When Abimilech sees Isaac and Rebekah being affectionate with each other, he realizes the truth and chastises Isaac.
Abraham's insistence on purity of the bloodline is also handed down to Isaac. When Esau (technically the eldest son) marries local Hittite women, his parents Isaac and Rebekah are not pleased. But Esau was responding to a dynamic that his parents created. Even though Esau was the elder son, his brother Jacob was the one that mommy loved best. In fact, Rebekah encouraged Jacob to steal a final blessing from dad while Esau was out hunting. If the story is to be believed, when the two brothers were younger, Jacob had demanded his older brother's birthright in exchange for food. When their father was about to die, mom apparently wanted to make sure the deal was sealed. So, in a feat of trickery that seems unbelievable, Jacob tricks Isaac into believing that he is Esau and Isaac blesses him.
Here is the weird bit: When Esau returns, Isaac oddly declares that he only had one blessing to give and now he's out. Even understanding that words have power and such, this seems like a strange claim to make. "I have two sons, but only one of you can receive my favor." How does that not equal atrocious parenting? To make matters worse, when Esau challenges Isaac on the "only one blessing" claim, Isaac basically curses him with a prophecy of a challenging and violent life. No wonder Esau gets back at mom and dad by marrying a few Canaanites.
As has been mentioned, it's a simple thing to write prophecies back into stories after the fact. If the history of these two brothers and the legacy they leave behind are the main themes of the story, then prophetic words from the mouth of their father make for good storytelling. This is essentially the way that an oral tradition works. One has to keep the listener interested in the story if they are to remember it and pass it down to their descendents, and foreshadowing is a very effective tool for maintaining interest. Still, the impression of Isaac as a father comes across as less than model parenting. And while it may still be the way some mothers behave, the intensity of Rebekah's favoritism (which leads her to encourage deceitfulness between her children) isn't much closer to an idealized picture of what a mother should be.
In any case, according to the story, Rebekah and Isaac are not happy with the idea of Jacob also marrying some local trash, and Rebekah is afraid that Esau may be angry enough to kill Jacob. Thus, Jacob is sent off to find a wife among his family's people and live with his uncle, Laban (Rebekah's brother). He finds two wives, as a matter of fact, and they are sisters. Rachel and Leah are actually Laban's daughters, and as the story goes, they are the first women Jacob sees. Since they are Rebekah's nieces, that makes them Jacob's first cousins. Since they are also Isaac's cousins twice removed, the two sisters are also Jacob's second cousins twice removed.
Laban doesn't make Jacob's courtship easy, either. Jacob wants to marry Rachel, and Laban demands seven years of labor in exchange. At the end of the seven years, Laban pulls a fast one and sends Leah instead of Rachel. (Laban and his sister must have learned from the same teacher.) But Jacob wants Rachel, so he works for another seven years and earns her hand as well. Their story has lessons of its own, but the basic theme in all of this can be seen in the best light as maintaining purity and in the worst light as insular and xenophobic. After all, holiness in its most basic definition means to be set apart.
The patriarchal family of the entire biblical narrative creates a belief system out of a fearful, insular culture in which deceit is practiced even within the closest familial relationships. All of the mandates and "shalt nots" in the scriptures can be traced back to this behavioral tradition. While we sometimes hear arguments that the culture of biblical times was different from modern-day culture, we rarely stop to think just how different it was. This family preferred marrying multiple cousins in order to maintain a purity apart from the influence of the rest of the world. Does that level of paranoia seem healthy on any level? Why do we accept rules and axioms based on fear and deception? Why is it impossible to conceive of a scenario in which brothers are encouraged to coexist peacefully and relatives don't deceive one another just to get their way? Aren't people capable of more than this?
That's a rhetorical question, of course. Some people don't believe people are capable of anything more than this. Some people believe that we are indeed deceitful, conniving, and scared at our very core, just like Abraham's family. The only person in the bloodline who seems to have any sense is Esau, who sees the relationships around him and decides he wants nothing to do with it. Although even he goes a bit overboard with marrying four Canaanite women just to spite his parents. Children of all ages are influenced by their parents' dramas, and the patriarchal line of Genesis is no different. Everyone has choices, though. We are only bound by the fears and beliefs of our parents as far as we want. We have the option to claim an identity based on trust and hope rather than fear and deception. And we don't have to go overboard like Esau in order to do it.
The desire for separateness brings up another issue for modern American society, however. If those who wish to honor biblical standards of behavior also were content to keep themselves apart from mainstream society, there are few people who would challenge them. Like Abimilech, leaders would most likely say what they could to make believers feel safe and let them do as they wished provided it didn't infringe upon the livelihood and well-being of others. However, some believers want to hold everyone to their standard. They want the whole of the population to be compelled to adhere to a standard that has more to do with fearfulness than it does with right and wrong. And many of them are quite willing to practice deception in order to get their way. While we can see a biblical precedent for this kind of behavior, this is not keeping oneself apart. This is not maintaining holiness. This is bullying, plain and simple. Frankly, I believe we are capable of better than that.