I was given up for adoption at birth and was adopted by my parents at 5 months old. My parents were honest with me from day one, I grew up knowing that I was adopted and feeling even more special because of it. Mom told me they picked me out of ALL the babies because they wanted ME, so when I was little I pictured it like the produce section of the grocery store only instead of tomatoes, cucumbers and lettuce, the bins were filled with babies they could chose from
. In reality, they were put on a waiting list and had to wait over 2 years before a baby became available. They even got a children's book that explained adoption that I begged mom to read to me time and time again when I was little. I remember on girl in grade school used to pick on me because I was adopted (I didn't hide the fact) and finally I had enough and told her "well, at least my parents chose me, your's were just stuck with you". It was just a fact of life to me and made me even more grateful for everything they did for me.
Now, if I would have found out when I was 30 something it would be a completely different story. I think this is the biggest mistake R could ever make if she wants any kind of relationship with your half sister. I took care of my mom for a year and a half while she died of cancer, and even knowing I was adopted, if someone from my birth family came forward during that time and wanted to start a relationship I would have been angry and insulted. It would be like someone saying "hey, you're about ready to lose the only family you have ever known, but guess what...we're here to replace her" like she was just disposable and someone else could just fill her shoes. Honestly, I probably would have punched them in the face.
Maybe a few years down the road your step-sister could handle the news and everything would be ok but definitely not right now. I know you can't control R's actions but please try to talk some sense into her (I know you've tried) before she adds more stress and heartache to that poor girl's life.