Mom's reaction was over the top. Was LW's as well? From the tone of the letter, it wouldn't surprise me. The real issue is the dynamic between these two. If their relationship has always been (verbally) violent, maybe the best solution is less contact.

The baby is largely a red herring. New parents are uptight (been there). Grandparents and sitters need to follow general advice, but parents also need to allow some leeway.