What is the ruling on preparing sweets for a wedding that includes music and songs in the marital home? Is the one who does so sinful, even if intending to honor their mother? And is there a sin incurred by refusing to prepare them, thus incurring the mother's anger?

There is no blame upon you for preparing sweets for your brother's wedding or any other occasion, even if it contains some objectionable elements like music, provided that your assistance does not involve aiding in disobedience to Allah. If a person can prevent or mitigate the objectionable elements in the celebration, there is no harm in attending. However, if they cannot, then it is not permissible for them to attend due to the involvement in aiding the objectionable acts. There is no harm in fulfilling lawful needs and requests made by the father, as long as it does not involve aiding in disobedience to Allah.