Sister creating disturbance with in-laws not taking care of son also

My co-sister is an illiterate woman causing for many disturbances in our family. Her rappo with inlaws and husband is not at all good. I couldn't understand her mindset. She is giving priority to other issues like quarreling with husband and inlaws than her son's physical and mental growth.She is not caring of his son.There is no good relationship between mother and child.He is not growing in a healthy atmosphere.She doesn't know the value of education.She usually leaves home for so many days to stay with her parents.She even don't care of her son,she never makes a call to her son also.We are facing problem with her behavior.What should we do? How to change her mindset and behavior? How to counsel her? Whom to approach?