"The style of the visit, and the shortness of it, were then felt to be decisive"

This superadded decision of character probably came from the Martins. It makes sense to me that they would glance at one another and decide unanimously "You had better go, then" and "Let me show you to the door."; saddened, comprehending, but knowing Harriet could not be easy if she stayed.
If Harriet had been obliged to decide for herself which friend to offend, she would have stayed distressed and dithering by the window.
It would be a familial resemblance to their brother's generous character if the girls were big enough to reciprocate with consideration for Harriet's feelings and Miss Woodhouse's convenience, even as they recieved this mean snub.