As a 19 year old, what type of legal action can I take on my 4 year old sister when I know her mother is unfit?

Okie. My friends little sister is not being taken care of properly. Her sister is 4 years old, and comes to her house or when my friend goes to see her little sister, she is smelly, doesn't take showers or baths very often, doesn't get enough fluids when she is just with her mom, and her mother sent her to daycare without underwear.. also her mother texts and drives with her kids in the car.. what can my friend do together knowing she is unfit.