What is my duty towards my mother and father to teach them the necessity of focusing on my sister's religious and moral upbringing in this era?

You must advise your parents gently and respectfully, reminding them of their responsibility towards their children, and cooperate with them in advising and teaching your sisters. It is incumbent upon a Muslim to teach their family members the affairs of their religion, and to enjoin upon them what is right and forbid them what is wrong. By doing so, you will have fulfilled your duty.