Is fasting required of me because I ate from my sister's food after swearing not to have dinner at her place?

The reference point for an oath is the intention of the one who swears it. If your intention with the oath was not to have dinner at your sister's house, but she brought the food to another location, or if you merely intended not to eat the dinner meal specifically at her place, then expiation (kaffarah (definition)) is not binding upon you. However, if your intention was to refrain from eating any food brought by your sister, and then you ate from it, you have broken your oath and expiation for an oath is due upon you: feeding ten poor people, or clothing them, or freeing a slave. If you cannot find any of these, then you must fast for three days, whether consecutively or separately. Allah Almighty says: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] - then a fast of three days. That is the expiation for oaths when you have sworn. But guard your oaths." (Al-Ma'idah: 89).