Is the failure to fulfill an oath such as, "I am of the religion of Jews and Christians if I do not return to her what she did to me," considered disbelief (kufr), and what should be done in this situation?

This reprehensible and forbidden statement that you uttered with the intention of emphasizing and urging yourself to take revenge on your sister does not lead to disbelief (kufr). You must repent from it by abandoning the sin, regretting having committed it, and resolving not to return to it. You are not obliged to do what you swore to do, and it is preferable to abandon it. Scholars have differed regarding the necessity of an expiation for an oath (kaffarat al-yameen) by abandoning what you swore to do. The more cautious approach is to make expiation by feeding ten poor persons or clothing them, or by fasting three days if you cannot find that.