I do get the impression that people of the Regency era (or perhaps just JA herself) had a different attitude about this than we do today. It's like Jane Bennet expecting to continue her friendship with Caroline Bingley after her break-up with Charles B. I think nowadays most people wouldn't want to spend more time than they could help with the sibling of the one who had broken their heart (unless they were close friends in their own right already) because it would bring up too many painful memories (not to mention the danger of meeting the ex). For JA, it seems as if the ex-lover was in one category, and his sister was in quite another. (I don't know if I'm making myself clear, but that's the best way I can express it.)