Okay, sorry for the delay...my bad.
This may have been discussed before, I'm not sure. But, here it is! Etta is not what she appears. She is either: 1) an observer 2) a scientifically modified human 3) a cyborg/robot 4) a combination of any of the above. There's a chance she's not Peter and Olivia's daughter at all. And, there's a chance that she is unaware that any of this could have happened to her and she really might believe she is Etta. Remember, she WAS taken. No one knows what happened to her between being the small child to now. We've seen what the observers do to "experiment" on "natives"...
Why can't she be read? Why does she always seem to be the one that is NOT killed? Why does she have contacts with the Observer go-betweens?
She's the Observers' pawn and no one knows it (in theory) but the Observers.
And, I could be completely off too...this is Fringe afterall