The Doctor followed Nora into the elevator. There was a silence that only comes with the nervous waiting for if the news you're about to hear is good or bad. The Doctor was running through options. She could theoretically get the TARDIS working, but the time stream and she wouldn't be able to go back. So if Nora's sister had been taken there'd be no option but try and rescue her before her wedding. 

That would be dealt with if it came to that. The other issue was how to get this group of aliens out of here. They were bad news. The Doctor had never actually fought them before. He...should she be saying he? Was always too late to stop them. She had always shown up in the aftermath. Planets destroyed. This was the first time she could fight them from the start. This time she'd save the planet. Earth was protected.

The Doctor looked back and forth between the sisters. Nora had dark hair, her sister was blonde. That was the main difference, but what about genetically. What if Nora had a genetic disposition for cancer, but her sister didn't have that trait. What if Evie had a genetic disposition toward depression, but Nora didn't. Their DNA would be similar, but they would also have variations. Those variations could be the difference between life and death...or blonde hair verses dark colored hair. That had to be what this was about!

"Nora! I think I understand. You're brunette and Evie isn't, she's blonde. You're genetically similar, compatible, but also far enough apart genetically that you could mix and match your DNA to get desirable traits! Did they take blood from you?" The Doctor was already checking Nora's arm, even though it was still around her sister, for any puncture wounds.