Well, when you started dating him, didn't he have his step-daughter at that time? And if she was a big part of his life when you two started dating, didn't you realize that someday, you'd have to kinda treat her like your own daughter, as in asking how her day was, acting like you care even if deep down you dont. If she's a teenager, she's typically going to be wrapped up in her own world so when she does talk to you, it's likely to be about herself, ya know?
If it's causing you this much stress, I'd tell you to explain how you feel to your boyfriend, quietly and when you're both calm. If he still doesn't understand, I wouldn't stay if I were you.
You are the new woman. No matter how long you've been with him, she's claimed him first, and he's her dad. I'm sure this is how she sees the situation! She is also most likely insecure, because he is her step father, instead of biological father. So you make her nervous, and she sees you as an outsider and will play manipulative games for attention and to make you appear unstable with the hopes that he'll eventually leave you. If she is a teen, then this is the case.
Be strong and talk to her about how you feel, or just be the bigger person and be kind to her.