I (19F) feel pressured to have contact with my half sister (18F) and I don't know what to do about it?

Back when my mom (40F) was pregnant with me (19F) my dad (40M) was cheating on her with my half sister's (18F) mom (40F). My mom and dad knew her from high school and she knew my mom and dad were married. My half sister's mom told my mom about the affair when she found out she was pregnant too. She wanted dad to herself and my mom was more than willing to let her take him because she knew he wasn't worth anything.

My half sister and I are only a few months apart in age. We went to the same school together until three years ago when my mom and I moved and I went to another school nearby. I did this because my dad was finally out of my life and I didn't want anything to do with my half sister anymore. We never got along. We always hated each other and fought all the time.

The reason why is largely because of dad. He married her mom but they were on and off again all the time because he tried for 15 years to get my mom back. I remember him showing up at our house and asking her to remarry him or he would show up and say he was in love with her and he was sorry and he made the biggest mistake of his life but she was the one for him. My mom even tried fake boyfriends to get rid of dad but he would show up every now and again. He even told my half sister and her mom he wanted my mom back and he wanted his family back. My half sister hated me for being the kid he didn't regret.

I think he didn't give a crap about either of us and never really cared about either of our mom's. I think my mom and him just had the kind of history he didn't like letting go of. If he loved her I would have no half sister. I'd have actual siblings but no halfs and no affairs would have ever happened.

My half sister has been violent toward me and I have been violent before too. She instigates and I wasn't always mature enough to walk away. So when I moved schools it was nice to be completely done with her. She enjoyed it at first too and apparently the longer we had nothing to do with each other the more she had a change of heart and she regretted things being so bad between us.

So she told our shared grandparents, who told me about it, and asked if we would sit down and talk. I said no and I told them I had no regrets about how we left things. They instead surprised me with her one time I was at their house for lunch and they pressured me to sit and talk to her. I argued back and refused to actually talk but while fighting my grandparents she told me she realizes she loves me and she wants a sisterly relationship. I ignored her and left because my grandparents were piling on the pressure. They told me I can't run from her and we need to work things out because our relationship should be super strong and it should be the good that comes from the bad. I told them I would have to want it or care about her but I don't. They said buried somewhere is love and I told them there's none buried. I told them I was never happier than when her and dad were out of my life.

They decided to apply more pressure because of what I said. They think I'm either lying or that I need to fix my feelings because it's wrong to truly be happy she's out of my life when we were both fucked over by dad. I feel so much pressure to have contact with her that I don't want.