Does sin include one who sits in a gathering where backbiting occurs without participating, and must he leave when backbiting begins, especially if he fears confrontation and hypocrisy when objecting or advising?

You must condemn backbiting among your female companions with wisdom and good counsel, and their mockery or accusations should not harm you. If they do not respond, then you must leave the gathering, because your continued presence implies approval of the sin. Denunciation by the heart is not achieved by mixing with those who commit evil; it is realized by leaving the place. If you are unable to leave, then only by the heart. Whoever is able to leave but contents themselves only with denunciation by the heart is sinful, for Allah Almighty says: "Indeed, you would then be like them." So your approval of the sin is a sin itself.