Proper form, well I don't know as to that, but what I believe is that Elizabeth wore the black ribbons as a simple statement of her being put out enough to acknowledge the death of the wife of Mr. Elliot. She would not be willing to go to any further lengths to "display" her mourning especially to a relative so unfortunately related by marriage and to pay anymore respect due than simple ribbons would allow. If I may be frank.... I'd say she was being a bit of a brat and nothing more.