Ahsan Allahu ilaykum, my mother always complains about my father and sister on phone. I live in a different country now and they are truly evildoers who have made her life terrible since several years. Is my mother venting to me about them considered backbiting?

Ustadh AbdulAziz al-Haqqan

This depends, if she is doing it just for the sake of doing it, then this is backbiting. However, if she talks in order to get her right back or seeks advice from someone who is wise, then she can do so to the extent that is needed.