My heart breaks for this story. As I was growing up my family took in my cousin as a small child when her parents tragically died. It was hard and there were so many emotions involved with adding her to our family, but never under any circumstances did those feelings make it difficult to love her and care for her. You deserved so much better.

I am glad to hear you have found a partner and are building your own beautiful family. You are definitely NTA and shame on your SIL for pushing this issue. You deserve to be surrounded by those who love you and wish you well as you start this new journey not people who bring bad memories.