What is the expiation required from someone who swore that his sister would not pass in front of him, but she did?

Your sister should have responded to you and fulfilled your oath, as long as no harm would result from it, according to the saying of the Prophet, peace and blessings be upon him: "The Messenger of Allah, peace and blessings be upon him, commanded us to do seven things," and he mentioned among them "fulfilling the oath of one who takes an oath." You are now liable for the expiation of breaking an oath, which is: feeding ten poor persons or clothing them or freeing a slave. If you cannot find [any of these], then fasting for three days, as stated in verse 89 of Surat Al-Ma'idah.