What should I do about my mother and sister, who have moved in with me and feel liberated from my father's strictness, and are engaging in activities that were forbidden to them, given my fear of the saying: "Each of you is a guardian and each of you is responsible for his flock"? And who is responsible for them now?

The one responsible for your mother and sister is your father. However, it is incumbent upon you to enjoin what is right and forbid what is wrong, and to strive to change the evil if they are committing a wrong.

Changing evil is obligatory upon you if you are the only one capable of removing it.

You should advise your mother and sister gently and politely, and threaten them that you will inform your father. If they do not stop the evil, then inform your father, unless informing him would lead to a greater harm.