Yeah, I agree, too. She
wants to be a Little Sister, but since that is impossible, she will be a BIG Sister. Her whole character is built around clinging to the "Sister" portion of her life.
Conjecturing about a family relation to her makes me think that, in order to pull that off, they would need to have a very detailed flashback or prequel sequences. What I mean is, if seeing Big Sister's face at the end is supposed to be a huge reveal, then you will have to
instantly recognize it. And it will only have an impact if they have spent significant time building that relationship up. That means tons of ghost sequences
or very clear, playable prequel sections with her....
Speaking of curve balls--and this is a bit OT for this thread--I wondered yesterday if Big Sis (or her master) might fight fire with fire as far as we are concerned. In other words, who's to say that that we were the
only prototype Big Daddy?

The developers have already said we would see new variants, but I think we assumed these were just different production models in other parts of the city. But I'm talking about really outrageous versions, kind of like us. They might have started with our model, scrapped it entirely for something different, then come back and said, "Well, the first once worked better....maybe if we power it down and make it more pliable...." Just a fun thought.