ESH. While you may be making the right decision to give up a child you clearly dislike the very existence of, it’s a poor way to repay your parents for their decision to adopt you and all of the pain and heartbreak they went through raising you. They might not have been perfect, but at least they took in an orphan rather than letting you bounce from one foster home to the next like so many other kids, getting abused over and over again. Like your sister could end up.

You mention one day having your own family. One thing others haven’t pointed out is how potential partners will react to the knowledge that you gave your sister up for adoption. Most folks (despite the NTA) comments here are going to view you as a monster for doing that. It’s the kind of action that will follow you around through friend groups and drive the guys most suitable to start a family with away. It’s not like you’re a naive teenager, you’re a grown adult, so at least for a few decades you will be judged very harshly for it.

Make the call that’s right to you, just realize that this may have to be your deepest, darkest secret for the rest of your life. Hiding it means it will be a time bomb too, waiting to emerge to toss your relationships into disarray at any point.

If you go the aunt route, be heavily involved. Heavily. Only way you can toe that line really.

There are no good answers here, clearly.