BB5: Project DNA
 

With the whole brother & sister thing, does anybody know how they did that? I can't work out how it's possible or was it just a miraculous coinsidence?

They most likely knew the theme beforehand and contacted family like parents etc.

I have no idea how they did the cow boy and Nakomis thing. My theory is that cowboy auditioned first (because he was the only one who thought he could have a long lost sister) and when they heard his story they set out to find Nakomis. I guess it was a coincidence they bother were fans of big brother lol.