What am I obligated to do regarding my oath, which I broke because I was unable to find an identical replacement for something I broke belonging to my sister? And is this considered breaking an oath?

Breaking an oath (al-Hinth) is violating the content of an oath, either by doing something one swore to abandon, or by abandoning something one swore to do. If a person swore to bring something but was unable to bring it, then he has broken his oath, and expiation (kaffarah) becomes obligatory upon him. The expiation for an oath is feeding ten poor persons, or clothing them, or freeing a slave; and whoever cannot find [the means to do so], then [it is] fasting for three days.