Is one who sits in a gathering of backbiting and slander, and is unable to leave or enjoin against evil, sinful?

You bear sin for that, and you must denounce the evil. If they do not respond to you, then you must leave them and not sit with them, as Allah Almighty says: "And when you see those who engage in [blasphemous] discourse concerning Our verses, then turn away from them until they enter into another conversation. And if Satan should cause you to forget, then do not remain after the recollection with the wrongdoing people." And His saying, the Almighty: "And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, then do not sit with them until they enter into another conversation. Indeed, in that case, you would be like them." And according to the saying of the Prophet, peace and blessings be upon him: "Whoever among you sees an evil, let him change it with his hand; if he is unable, then with his tongue; if he is unable, then with his heart, and that is the weakest of faith."