I think to focus on 'religion' in analyzing these kinds of situations is not that helpful because what most people think of as religion is a version of Christianity. Christianity is predominantly a patriarchal family system like a lot of other religions. Religion doesn't have to be patriarchal but it has been connected up with patriarchy in the West for many centuries, just like Islam.
Whatever 'religion' developed alongside this matriarchal non-father based family system would be quite different from what we know as religion today. The problems that you see in regards to homosexuality, bisexuality, open marriage etc and trace back to religion, I would trace back to patriarchy and it's basic evolutionary problem of the males being sure that their female sexual partners are not impregnated by another males resulting in the father raising a non-genetic offspring.
In this instance, the brothers don't care who f**ks their sister because the offspring is guaranteed to have a genetic connection to the brother.
Despite our inability to reproduce these kinds of families in our world in short order, its an important alternative because there are a lot of people in this world, some of whom have the ear of important Supreme Court justices, who think that the patriarchal family is the only viable child raising unit in a culture.

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