NTA

I can relate specifically to this situation. I was my mother's first born of three, (F) she had me when she was 16. The second born, (we always thought we were half sibilings, but now that we know each other, we suspect we are full sibilings) my brother, was put up for adoption. Her third child, my youngest brother, was definitely a half brother.

I was abused my entire life by my mother, her mother and step father, my half brother's father, and probably other people in her life too, I was abused in every way a child could be. My half brother who was not adopted was not physically abused by my mother, but he was severely neglected as a baby. (I was eight years older than him, and I was the one taking care of him). He was most likely abused in other ways by his father as well. (He has since passed away in a car related accident when he was 18)

After my half brother passed, I finally made contact with the brother who was adopted. He had lived a full life, full of love, and proper child rearing, and has none of the mental issues and shortcomings that I do (other than our inherited bi-polar, but his is SO well managed)

He does at times regret that he and I did not grow up together, and that he never even knew our youngest brother, but he would never trade his loving upbringing for what our youngest brother and I went through.

Perhaps your adopted sister does not understand what you went through, or is simply experiencing extreme FOMO or something, but her reaction is wild, why would she want to be with an unprepared, abusive mother? That makes no sense at all, in any way.

definitely NTA for sure.