Is it permissible to attend my sister's wedding, to which my friends are invited, and eat of its food, knowing that the groom works in a usurious bank, and that he intends to change his job, but my family argues that he will search for another job, and that my absence will anger my family?

Working in usurious banks is unlawful (haram) and is not made permissible by the need to marry or assist family members. Therefore, it is not permissible to marry off a suitor who works in such banks. As for attending the wedding and inviting friends, it depends on the expediency: if refraining from attending changes the reprehensible act, then it is more appropriate; otherwise, one should attend with advice, to avoid severing ties of kinship. As for eating from the wedding food, it is not permissible if the food or the money used to purchase it is unlawful.