Would I bear the sins of my sisters if I advised them that watching TV series is forbidden, and then they watched them?

You must continue advising your sisters to stay away from watching TV series that contain forbidden scenes, clarify the ruling of the Shari'ah to them, and encourage them to watch beneficial programs and purposeful channels as an alternative, while providing them with recreational and cultural alternatives.

If you do so, there is no sin upon you. However, do not stop at saying, "It's none of my business." Rather, strive to advise and denounce with your tongue, heart, and hand as much as possible, while observing good manners and gentle speech, and frequently supplicate for their guidance.