Slen tiid vo?

Is it possible that siblings can both be Dragonborns? Also in the event of an alien invasion can the Nirnians beat them? :P

That doesnt seem like it breaks any kind of lore. Thats just someone changing sides because of a change of heart.

Nope. Different souls.

Also, according to the widely accepted, and completely flawed cosmos situation in TES, there are no aliens. E.T.s I mean. Illegal aliens crossing borders, sure.

So the Dragonborn isnt anyones soul in particular? Its just a reincarnation? Right?

Why does space have to be confusing in this game...

The theory its the soul being in a human. Some folks say its the blood. Maybe, identical twins would fly.

Because it's built on false hoods that the Altmer formulated to justify their ancestor worship. 

As an Altmer enthusiast I agree with you. The Dominion love to spread their propaganda with pride.