What is the ruling on an unveiled girl sitting with her mother and her 12-and-a-half-year-old brother in her aunt's house, which includes her husband, daughter, and son? And what is the ruling on eating and sitting with them? And what is the ruling on laughing when hearing backbiting from them?

The fundamental principle in family gatherings is the segregation of men and women, due to the fear of fitna (temptation/strife).

If the family gathers without suspicion, while adhering to the شرع (Islamic law) by avoiding seclusion, observing hijab, and not speaking in a soft, alluring manner, then there is no harm. Some scholars have permitted a woman to eat with a non-mahram man when there is no fear of fitna.

It is obligatory to object to anyone who engages in forbidden backbiting. If you are unable to do so, or if your objection is not accepted, then it is obligatory to leave the gathering.