Sister was adopted- found her, Advice needed!!

Hi all, Thank you all in advance for your advice! My younger sister was adopted from birth. I was not in contact with my mother at the time, so I had no idea I had a sister until much later. (You could say I was adopted by a family member) I later found out I had a sister and immediately wanted to meet her but knew nothing about her. I recently learned her name and found her and her family quite easily on facebook. I'm older now (21) but she is still a teenager, and I was unsure if she even knew of her adoption. I decided it would be for the best to reach out to her mom instead. I asked about my sister, if I could contact her, if she knew about me/ the adoption. In the end, she knows she is adopted, but not much else and the adoptive family really would rather no contact between us. This has left me feeling very sad and almost caged/controlled. I dont want to do anything to hurt my sister, and if shes in a difficult time in her life i dont want to over complicate things. But I am worried that her well being is not the real reason her parents dont want her to have contact with me. By what she said, it seems that her main reasoning is that she has a loving family and doesnt need me. What do you guys think? How do I calm down her family so they realize that I could never ever think to replace them? I really want to get to know my sister, especially because Im getting married soon. Thanks!