What are the rulings that apply when a wife says to her husband, "You are forbidden to me, just as my brother is forbidden to me," and does her husband become forbidden to her, and is this considered zihar from her?

This is Dhihar (a pre-Islamic form of divorce) from the wife, and it is a reprehensible and false statement. Its ruling is similar to that of a strong oath, and its expiation is the expiation for an oath. So, whoever forbids something upon themselves that Allah has made lawful for them, their expiation is to feed ten poor people, or clothe them, or free a slave. If one cannot find [these], then they must fast for three days.