I resent my dad for marrying a woman who had kids with complex needs after my mom died

I lost my mom at a young age and my dad was remarried less than 18 months later. His wife had three children who all had their own complex needs. One has level three autism and required a lot of care and services, another was neurotypical but had a lot of health conditions and the last child had a lot of complex disabilities and could not walk, talk, speak, could not eat or breathe without assistance and was hospitalized frequently. None of the kids had a father in their life and because of that I was expected to play the role of an older caring brother who had their back when their mom was focused on one or two of the others. I was expected to be understanding of my dad no longer having time for me. I was expected to love the woman he married and look at her as a mother even though I had a mom and this woman had to focus all her time on her kids and maybe a little with my dad when possible. I had to give up on having my own room and sleeping through the night as well. And I was expected to be extra close to the child with health issues but was neurotypical because we were close in age and they would often be alone in the hospital so I was expected to visit every day when they were in and keep them company and be by their side for certain appointments or scheduled treatments. My dad would always say we did what we could for family. He would be angry at me whenever I told him I didn't want us to be a family and they weren't worth giving up what we had. There was a time he asked me why I had to be so selfish and why I couldn't embrace having more people to love. But I never loved a single one of them. They felt like a burden and I resented my dad for signing us both up for that. I felt like it would have been easier if he and mom had been divorced and he married his wife because at least I would have my mom but I didn't. My relationship with my dad today is minimal. He has no idea how deep my resentment goes but there is tension because I have made it clear that I will never be a caregiver for any of his wife's children and I will not be on call support for them either.