Is the questioner sinful if she leaves her sisters to prepare food for a brother who curses the religion when dishes he dislikes are served?

It is permissible to shun one who insults religion if doing so serves a beneficial purpose. If the brother’s insulting of religion occurs when preparing food in a certain way, then the questioner and her sisters should avoid that, lest it lead to the insulting of religion. This is based on Allah Almighty’s saying: "And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge," and on the Prophet’s (peace be upon him) saying: "Indeed, among the greatest major sins is for a man to curse his parents." It was said: "O Messenger of Allah, and how does a man curse his parents?" He said: "He insults another man's father, so he insults his father, and he insults his mother." The questioner is not held accountable for her brother's insults to religion in her absence, for not every means to something forbidden is absolutely prohibited. Means are of three types: those unanimously agreed upon to be blocked, those unanimously agreed upon not to be blocked, and those about which there is disagreement.