So, I have this idea.

Women, you see, hold incredible power. Because they more-or-less control access to reproductive resources, they also control what it means to be a human being. No human beings exist without the consent of a woman.

This situation is less than ideal if you're a man. You're only able to affect the way the world is for as long as you live. No matter what you accomplish in life, if you can't convince a woman to bear you children, what makes you what you are will die out forever. This drives male behavior throughout their lives.

Whereas sons can pass on a mother's X chromosome, daughters can only help their father to be genetically successful if they choose to mate with a man who is genetically close to their father.

I think the big religions are a male reproductive strategy. If they can convince women to only reproduce with members of their tribe, then they'll increase the odds that their genetically close y-chromosomes will have offspring.

In your case, your male relatives marrying outside the religion increase the amount of reproductive resources available to males of your 'tribe' while decreasing the amount of resources available to males of an outside 'tribe.' If you, however, made your reproductive resources available to a male outside your 'tribe' then your close male relatives would lose a potential avenue of reproductive success, while outsiders simultaneously became more successful. So, your male relatives have a vested interest in your choice of mates.

'Islam,' like other religions, is an unconscious way to convey these kinds of genetic strategies, and does not require the participants to have any knowledge of genetics. By stipulating extreme social control mechanisms on women, and by excusing these control mechanisms as coming from a power higher than male purview, males are able to guide female reproductive resources to serve their interests, without alerting the suspicion of women.