While I don't like it as much as Rosalie, it is an ok name and the first association w/ the name that comes to my mind is Rosa Parks, which is a positive one... a woman w/ some chutzpah.
Since roses come in many colours besides red, I don't consider Scarlett and Rosa too much red for 1 sibset. They sound pleasant enough together... Plus, both girls would have their own lives and personalities, so it's not that big an issue anyway if there's a slight colour relation... As long as you don't have another child named Red or Rouge, I think you'll be ok on that.