AITA for refusing to attend therapy with my family so I can try to have a good relationship with my half sister?

When I (17m) was 8 my dad found out he had a 10 year old daughter. There was no cheating. My dad started dating my mom after he broke up with half sister's mom. She moved back home and didn't tell my dad she was pregnant. My parents met and had me pretty quickly afterward. Dad's ex only told him about their kid because she was asking questions and wanted to know dad. My dad did a DNA test and then he met her and he spent some time with her and then he introduced me and my mom and then she started coming to our house.

My half sister was so jealous of me. She hated me. She hated that dad had raised me and she was new. She treated me like shit and said over and over how she didn't want a brother. It was all weird to me too and I hadn't exactly wanted a sibling either but didn't get a say in it anymore than she did. I'm aware I had the benefit of knowing dad and not going through what she did. But to be honest I was tired of dealing with her and liked when she went back home. I'd have been happy to never see her again.

Three years ago she decided she wasn't going to come to our house anymore because she didn't want a relationship with me and my mom. My dad was upset by her choice but carried on a relationship with her outside our home.

She's 19 now and her and dad have talked and he told her they couldn't have the relationship she wanted if she never wants to be around me and mom, because he won't abandon us every holiday to be with her and he won't exclude us from his birthdays or other milestones and achievements.

She considered it for a while and now she wants to try and make things work but doesn't know how. My parents talked about it and dad asked her about family therapy and she agreed. My parents talked to me about it and I said no. Dad looked crushed by my answer and how quick and firm I was. He told me this could be the chance for us to have a relationship and he was all like "don't you want to have a good relationship with your sister?" My mom asked how I felt and I told them I felt like she wasn't a real sister and I didn't love her like siblings do. I said she came into my life when I was 8 and wanted nothing to do with me. I said I never bonded with her or grew to like her. I said I wouldn't ignore her if she did start showing up but I couldn't imagine us being close like siblings who grow up together or come from the same family. I said she was a kid and had a lot to work through. But that didn't mean I had cared about her all this time. I told dad she's his kid too and I don't want to stop him from having a relationship with her and I won't treat her like shit if she's around. But I said she's basically a stranger who bullied me before.

My dad said he understood but I could see he was upset about it and I heard my parents talking after and he said he was so sad it had all ended up this way. But that he didn't blame me. I know I could try for him which makes me wonder if I'm TA. I just really don't want to work on things with this girl. Does that make me an asshole?