I agree with your sister, FWIW. P'a is bigger than US-standard green onion (scallion) but differently shaped from leek or "spring onion" (latter is often red-tinged around here, unlike conventional scallions). FWIW, my mother says p'a is none of the varieties for which Kitazawa sells seed!

Against that, distinguishing flat chives from scallions is easy. What I have trouble with is flat chives versus garlic chives.