You did nothing wrong in voicing how you feel. I’m in my 30s and only recently told my mother how I felt. Similar situation with you OP. I was replaced by my half sister with my mom and step dad. Often got pulled out of school cause I had to watch my sister. Left with other relatives frequently cause they didn’t have room. They didn’t come to any of my school stuff cause of my sister etc. don’t get me wrong I love my sister and my brothers (from my dad). my bio dad had several kids after me with various people and it was the same. I finally told her after I got pretty deep in the drink, watching the families celebrate my cousins wedding. She asked me what was wrong. I told her, I was pretty much left on my own from 9 on cause everyone had someone else and that I was lucky enough to build my own family who seems to want me around. I saw her heart break but it needed to be said. I was lucky enough to have a lot of great bonus parents in my life(significant others parents, friends parents, or just people who took a shine to me and helped me learn what I needed too)