There are couple of reasons people do that .
1 . I think the most important one is that they attribute the strength of the family to stay united . Their concept of staying united is to keep on marrying inside the family and all the family will stay together .
2 . Marrying inside the family also help in preventing further division of wealth . Like traditionally people had land that gets smaller and smaller if they marry outside the family .
3 . It is hard to trust people outside the family . Parents feel more secure in marrying their daughters to someone from inside the family , as they think that they will treat her better . Another think related to it is the family influence on the relationship between husband and wife .
I think these are few reason why people marry in their family . But as time is changing so is this tradition .
There is nothing wrong in of two sisters marrying in the same household, but there is big danger in cross marrying (Both family take a girl of each family).
In case relationship does not work for any couple, the relationship of other couple used by family as blackmailing point.
So if you are marrying in a traditional family with strong inter family bond please be careful.