Is our discussion of our father's family who mistreat and oppress us, and mentioning their faults in response to their insults against us, considered backbiting (gheebah)?

Backbiting is one of the major prohibitions, but it is permissible out of necessity for a Sharia (definition)-compliant benefit, including seeking redress. This is based on the Almighty’s saying: "Allah does not like the public mention of evil except by one who has been wronged." Scholars have mentioned six situations in which backbiting is permissible, one of which is seeking redress. Therefore, there is no harm in speaking about the injustice inflicted upon you by your father's family, provided that you only mention their faults that relate to their injustice towards you.