Not only might they not reflect the behavior of breast cancer in its own milieu(the human patient with the special changes present in the tissues which surround the cancer and their circulating and local hormonal, immune and other substances) and due to changes in the cell lines caused by the many years they have adapted to non-physiologic surroundings (growing on strange materials in petri dishes with strange "cocktails" of substances found to be required for perpetuating them in such a strange environment, let alone changes caused by implantation in a foreign species (even those bred to have deficient immune systems), but also because few readers of articles bother to check the characteristics of the cell lines discussed--and breast cancer is not one disease but a mixture of many, like an exotic fruit salad, you can't compare apples with oranges with kiwis, guavas or passionfruits.
Here is a source of some cell lines discussed according to their ER and her2 status. I believe I may have cited an even better list about three years ago (search function may help)
http://www.geocities.com/m.lacroix/bcrt4.htm
This paper presents MDA MB-436 as an ER- her2- breast cancer cell line, hence the dangers inherent in interpreting the Zoledronic acid,anthracycline
combination papers as applicable for her2+ breast cancers, whether ER+ like your mother's or ER-