How can a young man advise his two sisters who are nominally non-Muslims, and who grew up in a non-Muslim society, and guide them to the right path? And is he sinful if they do not respond to his advice?

You must gently advise your father and explain his responsibility towards his two daughters, and that he should command them with the laws of the religion, and that he will be held accountable for this on the Day of Judgment, in accordance with the Almighty's saying: (O YOU WHO HAVE BELIEVED, PROTECT YOURSELVES AND YOUR FAMILIES FROM A FIRE WHOSE FUEL IS PEOPLE AND STONES).

You should also explain the virtues of Islam to your sisters, that it is a religion of mercy and justice, and that there is no happiness for them in this world or the Hereafter except by adhering to it. You should be a good role model for them in your behavior and actions, and endear yourselves to them with gifts. If they respond, then praise be to Allah, otherwise, there is no sin upon you after exerting your effort in advising them.