Charging a 5 yr old with murder is insane. She probably did intend to kill the younger sibling. But “intention” in a 5 yr old (as well as their understanding of death) is a little hard to define, particularly since this child probably came from a family where ethics, morality and right-and-wrong are all a bit on the fuzzy side.
It should be criminal negligence for the mother and possibly the 16 yr old, and the 5 yr old should be taken out of the home permanently and sent some place where she might learn to be a decent human being.
I agree with you completely.