I feel your pain, what you went through was(and still is) pretty brutal. When divorcing, parents rarely understand how traumatic it can be for children to have their family split all of a sudden, and being literally uprooted like you is beyond messed up. That's too many changes at once and you definitely could have used therapy to help cope with it all(its not too late to seek help btw). About your little step sister though. She is the one person that actually tried to welcome you in and tried to make you feel like you were part of the family. She showed you love when she could have kept it moving just like her older brothers/sisters pls do not overlook this. You being practically kidnapped by your dad, while fucked up in itself, is in no way shape or form her fault. Im not saying you should spend all your time with her and be all brotherly of a sudden-especially if you dont feel like it- but just be a decent human being to the only person that made some effort to make you feel like you belong. Acknowledge her, and don't push her away when she asks you questions, just hang out with her every once in a while. Besides from what i understand she's just a little kid, you can just watch cartoons with her or play a video game with her from time to time and it'll mean a lot to her while not taking much time out of your day. Love and light stranger, hang in there it'll get better eventually, you can always pm me if you wanna vent or whatever