It's a question from a non-Muslim - If we go back to Adam and Eve, don't we all have the same father and mother... and sister.. Then whom do we marry? Did spouses for Khan, Dsouza, or Singh drop from the sky? Where did different different families come from?

The question is, if any Muslim marries his or her cousin, it is said they marry their sister or brother. If Adam and Eve were the only people on the earth, then how did we become we families?

Well, first of all.. I'm Muslim, let me tell you what I do know...
Something's can't be discussed in a logical way, you can't just tell us many things about how the universe is been operated.
If we depended on the fact of everyone has a father, it would be like who's the first the egg or the chicken, and probably it won't solve the issue.
About that, there is a god who created many things that we are one of them, it's like if you have a farm or whatever you always start it with one of each kind, then the distribution goes over :)

Point 1: Adam and Eve are the parents of all human beings.

Point 2:
At every point of time, God sent messengers with a message, that needs to be followed for that particular time only. Eve gave birth to twins 20 times. By the grace of God, every time, she gave birth to a son and daughter together.
The message of God to be followed at that time was to allow the marriages only between siblings who were not born together.

Point 3: Regarding Khan, DSouza and Singh;
When the children of Adam moved to different places on the earth, their colour, race, etc changed due to the climatic conditions, and they adopted various names according to the existing culture.

Finally, we are now in age of the Last Prophet Muhammad pbuh and the last revelation i.e. The Holy Quran; and this age continues till the last day(Qayamah). Hence, today all the human beings, regarless of their surnames, colors, races, etc must believe and follow this last messenger and message.

And Allah is All knowing, with Complete knowledge