Does my step sister have to right to be mad at me?

So my step sister's step mom is my estranged mom. I came to a new school because I lived with my grandma but then she had to move to Canada so I'm living with my dad. I started dating this boy on my dads team, he really cute first person to talk to me. We've been dating for 2 weeks I knew I was in the same school as my step sis but I didn't know that the boy Im dating was her ex boyfriend. He didn't even know she was my sister. I've been trying to be friends with her and she's making it difficult now she hates my guts she makes rumor about me makes fun of me. And what's weird she cheated on him with her now boyfriend. Does she even have the right to be mad at me?