Would I be sinful merely by sitting in a gathering where backbiting is practiced, even if I were talking to my sisters and not participating in the conversation or listening to them?

Whoever hears unlawful backbiting must silence the speaker. If the speaker does not desist, it becomes obligatory to leave the gathering, unless there is a need or necessity to stay. Otherwise, one is exposed to the sin of condoning backbiting. Imam al-Nawawi explained that just as it is forbidden for the backbiter to utter it, it is forbidden for the listener to hear and condone it. Therefore, one must forbid it if there is no fear of harm. Otherwise, one should disapprove in their heart and leave the gathering if possible, or interrupt the backbiting with another topic. If none of these are done, one commits a sin. If one is compelled to remain and is unable to forbid it, it is forbidden to listen; rather, one should remember Allah or think about something else. If one is able to leave, it becomes obligatory to do so.