How would YOU handle this situation?

My mom and I have never had the perfect relationship. She’s told me times she’s hated me, wish I was dead, etc. I’ve returned the favor in my past by being disrespectful; if you hate me then I hate you. Within the past year, our relationship has gotten so much better, I’m very open with her now, we respect each other more, tell each other we love and care about one another, etc. she has this “cousin” on her side of the family that always disrespects me when she calls my mom. Today she totally crossed the line with me. In the past she’s spoken to me like I’m her child and my mom has brushed it off. Today I came into the house happy and I spoke to her on the phone with a simple “Hi”. This BITCH had the nerve to say “Have you been behaving yourself? It’s a simple yes ma’am or no ma’am?” My mom told me to ignore her AGAIN. I’m 19, I’m growing physically and emotionally as well as mentally. This lady has disrespected me since I was 17.