CAMBRIDGE — The nanny and infant brightened the neighborhood near Harvard Square, a smiling tandem who routinely waved at shopkeepers around the corner from the modest apartment where Aisling McCarthy Brady cared for Rehma Sabir.

Now, Rehma is dead, the victim of a catastrophic head injury, prosecutors say. Brady has been charged with violently assaulting the baby two days before she died. The case has stunned and saddened those who knew them.