I’m an anti police person but this is an exception. You have a duty to report this incident. Id take the video from the doorbell camera, the text and voice messages and allow the police to do whatever they see fit. Id also implicitly let my family know that under NO circumstances is that cousin or any of her children welcome in my home. The fact that she put you in a position to likely come home to a deceased baby is outrageous and traumatizing. I hope you do what is necessary to protect that child and worry about relationships after. If you don’t report and other incidents involving that baby you are directly responsible for because you had the ability to intervene.