And of course with dogs and cats. Creating the long extinct species of Dumans and Humats.

sorry, don't take that serious, watched way too much comedy central today

So we had incest and beastiality going on.

ahh, those were the days

Don't forget the polygamy and whatever you call sex with angels and nephilim.

I don't even think their is a word for that but I will see what I can find.

No thats our most recent common ancestor, not a first couple/person, there is a big difference. Its also funny as our most recent common ancestor of each sex lived 80000 years apart

Please read this page for more info:
Most recent common ancestor - Wikipedia, the free encyclopedia

As a Muslim, I believe in Islam's answer:
Islam forbids marriage between the brothers and the sisters. Allah Says (interpretation of meaning): {Forbidden to you (for marriage) are: your mothers, your daughters, your sisters,….}[4:23].
The above ruling is in our Shariah and every Prophet has his own Shariah. Allah Says (interpretation of meaning): {…To each among you, We have prescribed a law and a clear way. …}[5:48]. Hence, it was lawful in the Shariah of Adam (Alaihi Salaam) that a man married a woman who was not his twin. It is reported in the Books of Tafseer like al-Tabri, al-Qurtubi, Ibn Katheer, al-Bidaya Wa al-Nihayah that every time Hawa (Eve) used to give birth of twins; a boy and a girl. Adam was ordered to marry his male child to his female one who was not the male’s twin.
Source: Islam Web
Another Fatwa: Source of generations after Adam

@Matodo: I dont get it, they are still brothers and sisters, right? What does them being twins have to do with it? The second translation-interpretation seems to only say that it is clear. (which is a bit redundant, saying something is clear does not mean it is so)
PS: About the Shariah, this thread might be interesting. Some of the comments might be a bit harsh, though. Please understand that this is because of the implications for the "Zimmis" in the link of the thread.

Then what was she to him then or did she just appear out of nowhere?

Well, I think that my previous reply contains the answer to your question
Well, I'm sorry because I didn't write the whole Ayat:

Allah Says (interpretation of meaning): {And We have sent down to you (O Muhammad (peace be upon him)) the Book (this Qur'an) in truth, confirming the Scripture that came before it and Muhaymin (trustworthy in highness and a witness) over it (old Scriptures). So judge among them by what Allah has revealed, and follow not their vain desires, diverging away from the truth that has come to you. To each among you, We have prescribed a law and a clear way. If Allah had willed, He would have made you one nation, but that (He) may test you in what He has given you; so compete in good deeds. The return of you (all) is to Allah; then He will inform you about that in which you used to differ} [5:48]

That still doesn't make sense.