What is the ruling on attending my sister's mixed wedding party where Islamic songs with music are played, and what should I do if my not going will sadden her?

If the sister who asked the question knows that her sister's wedding will include music, mixing, or other forbidden acts, it is not permissible for her to attend the celebration unless she intends to denounce them and knows, or strongly believes, that the forbidden act will cease due to her denunciation. Otherwise, attendance is not permissible.