Must I offer expiation after I swore to my sister that she would not eat pizza with us, then I unintentionally took a slice from my brother and ate it?

If you intended with your oath that your sister should never eat from this food, then you have broken your oath and an expiation for the oath is incumbent upon you. The expiation becomes incumbent here if you were an adult at the time of the oath. However, if the oath was taken before puberty, then no expiation is due from you.