This is an important topic. But, why in some communities, marriage among close relatives takes place? Before we go into the answer to this question, let us see what happens in a marriage:
Two persons get married on a fine day. That day, the woman is told she has to live with her husband and his parents. And she has to do that, based on the way the society is. But then, she finds she is not happy in her life with her husband and her in-laws. She finds those persons have totally different outlook in life and totally different customs and some of which her own parents had said were taboos. This is all painful to her because the way they all behave is unfamiliar to her and neither has she any idea how to familiarize herself in this different situation. She is also forced to accept those customs. When she tells her parents, those persons are doing something she was forbidden to do, her parents have no answer to her question. At times, they tell something to justify her husband's family but she would not be satisfied by that answer and she is confused and depressed. No one is able to answer her questions - her husband or his family or her own family. What could she do?
If she is mature enough to take care of herself in this situation or her parents train her to become mature things definitely work. In that case, she will learn more about her husband's family and understand them and she takes tender care of herself also.
But this is not the way, every young woman could be and neither her family could be. The family must not become guilty not to answer her pains.
Suppose two cousins get married. It is possible they both have the same pair of grand parents. The outlook of these two persons could be similar. And so the wife will not be at sea when she is with her husband as she knows him since his childhood and she is familiar with him. She can take her own time to get mature. His own parents and relatives also know her very well and she also knows them well. And definitely, we see parents are more loving to their nephews or nieces rather than their own off springs and so the marriage could succeed with less effort. The woman need not suffer silently and she needs to listen to something which confuses her.
Thus there are good reasons why we have marriages among close relatives. It is definitely possible the woman could find all at sea if she is sent to a different state or is sent to the town from the village. After all a woman is married to get a future protector and provider for her. And she must become insecure because the man whom she marries is somewhat immature.
When I say all this, I am overlooking the biological aspects of the marriage among close relatives.But one could see how they brought this type of custom.
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