Is it legal for my mother who has extreme (at times) signs of dementia care for an infant?

There's 'nothing we can do' insofar as forcing your mother to go to the doctor or get care for her dementia, or even a diagnosis, if she flat out refuses. That's true. BUT, there certainly IS something the mother of the infant can do about not letting the woman babysit 'all day long' for the baby, for godsake! Have we all lost our minds here? You say, "I am in disbelief that my sister is allowing her granddaughter to be watched all day by her. It’s terrifying to me what could happen." It's up to the child's MOTHER to say NO about the demented great grandmother babysitting, not your sister.

Maybe you are just getting the wrong information here b/c it's really too much to believe any mother in her right mind would allow an elder with extreme dementia to 'babysit an infant all day long'. It IS against the law as others have pointed out, it's called child endangerment, and if this is truly happening, the mother of the infant has to be reported for it. The child can die and the mother will go to prison for it, in reality, for knowingly hiring a babysitter who has extreme dementia!

Put it to her like that (the baby's mother), and say that YOU will report her to the authorities for CHILD ENDANGERMENT and see what she has to say!

It's mind boggling to me that nobody has the cajones to stand up to a demented elder and would rather put her infant's LIFE at risk instead! Appalling.