What is the ruling on attending a mixed-gender engagement party (Katb El-Kitab) of one's sister, which features Islamic music and songs accompanied by musical instruments and ululations, with the option of leaving after the Katb El-Kitab or waiting outside the party so as not to displease the family?

There is no objection to attending the marriage contract (aqd an-nikah (definition)) if it is free from forbidden acts such as music and intermingling. As for attending a celebration that includes forbidden acts, it is not permissible except with the intention of condemning the wrongdoing. If a person cannot change the wrongdoing, then it is not permissible for him to attend, and he should advise those organizing the celebration to remove the forbidden acts. There is no such thing as Islamic music. If it is possible to avoid forbidden acts and not displease one's family by attending the marriage contract and then leaving, that is good. Otherwise, the pleasure of the Creator takes precedence over the pleasure of the creation.