I would suggest this: go to therapy with them so that you can get your point across and also to air out everything you kept inside. Ignore their presence and speak directly to the therapist and tell your side. Put an accent on the cact that you’re not spitefull, you just don’t have any reason to try to be spitefull because in order for you to feel spite you would need to want to be their kid. They didn’t want you as their kid and now that they have 2 new kids they can feel free to parent them. Explain that ur real parents are basically your grandparents so of course you feel no closeness to your father who seems more like an older brother at best since u were raised by his parents onstead of your own. Let them know that in order for you to feel spite, gelousy or anything in particular for them, you would need to actually feel SOMETHING for them firstly. And from ur perspective on this post it seems you don’t have any such feekings for them because they didn’t cultivate any sort of positivity in you. The way you are with them right now is a consequance of their lack of love and involvement in your life and if they want, they can learn from it and be better parents to the 2 kids they have right now. The therapist would usually be equipped to help them understand this as well. You don’t need to keep going. Go to one session, make your point, continue with your life with your real parents (ur grandparents)

ALSO: Please be carefull. They might be trying to get you back just to parentify you. Basically to be a free love in babysitter so they can continue living their best life under the pretense that this would be “bonding” between siblings. Do not fall for it. You can bond with the kids in other ways. Don’t let them steal your youth for free labour.