I believe that Dr. Kureha was the former doctor of the Rocks Pirates.
Dr. Kureha possesses genius skills and immense medical knowledge as one of the finest doctors in the world.
Outside of her medical expertise, she possesses a significantly high level of superhuman strength and speed to match her vast longevity. And the fact that she is 141 years old is another reason why this theory makes more sense. She seems very confident and cocky for an old woman, which leads me to believe that she used to be a pirate. Furthermore, she dresses like a rock/punk girl and has the appearance of a “witch”.
These characteristcs fit for somebody who was part of the greatest pirate crew in the world.
Also, at the end of the Drum Island arc, one of Dalton’s subordinates comes with Luffy’s wanted poster, Dalton gives it to Dr. Kureha and she notices that Luffy has the D. in his name. She definetely KNOWS something about the Will of D. (and probably about Rocks D. Xebec) and I’m confident that she found out about this information by being part of the Rocks Pirates crews and having an encounter with Monkey D. Garp and Gol D. Roger.