Grandma did not kill the baby

You have to remember that the Amish serve the Lord. If you put yourself in the mother's shoes, yes she was there to cut the cord. I think if the baby was not already dying from the disease, Grandma would have found a way to get the baby off the farm and into loving hands.

So why the dramatic ending. Because she could have saved her daughter from the trauma of a trial by jury. She could not speak because she needed to be a conduit between Katie's father and Katie. If he knew that his wife knew anything at all about the pregnancy, she would have no sway over him and he would order Katie off the farm and she would lose her child.

The Amish civilization works because of the structure and dynamics of its core beliefs. If Sarah had actually killed that baby, it would have created a paradox that simply does not work and the family structure would implode from the inside out.

I know as a mother, I would go to my grave keeping my daughter's secrets. This is the only thing she is guilty of. This is why she gave the shears to Ellie. So that she can continue to keep their family afloat.