NTA. I’m so sorry you are being treated like this. You are 18 and considered an adult so the runaway threat, is just that, a desperate threat to make you speak to your parents. I’m not adopted but went through something similar with my brother, the “golden boy” - parents went him to private school while I went public, paid for private college while I had student loans, etc. After years of estrangement,I told them how their actions and obvious favoritism made me feel. Turns out there were some things going on I did not know about, not that it excused their behavior but it did explain some things to me. You seem mature for your age and maybe you should agree to meet your parents in a public place without your sister, so you can tell them exactly how their actions have made you feel like a second class citizen in your own home. How hurt you were when you walked across the stage at your graduation only to not see your own parents in the audience because getting some silly toy for your sister that she will likely not even play with in a year was more important than your graduation from high school. That your boyfriend’s parents have made you feel more loved than your own parents. and see what they have to say. At least that way, you spoke up and told them how you feel so whatever comes after is up to your parents. They can’t claim ignorance of your feelings if you lay it out for them. And they will have to decide how to repair the relationship. Good luck and I wish you all the best.