help me out here 😩

Currently in a relationship with my partner and we are going to be getting married soon! 🧿

But there is an issue…. His sister.

Long story short his sister is 15 and I can’t stand her. Her parents have given up on raising her entirely, she has no respect, smokes and drinks everyday and leave and runs out the house and doesn’t come home till 3am or so. I get it some teens are like that but she has disrespected me, lied about me to my partners parents to the point where they believe her fake stories and now they don’t like me, she’s tried to ruin my relationship with my partner as well and she’s disrespected my own family.

I can’t stand her, my partner just brought her up in conversation today and I let my partner know that I will never respect her, or like her again until she apologies and I see some change from her. He got mad at me and took her side. He continued to tell me she’s not wrong and that I’m wrong here. Hung up in my face and then proceeded to silence me.

Am I wrong here?

What do I do?

Honestly I don’t want to be in a relationship if this is constantly going to be an issue with his family towards me, but I adore him.