There are other factors involved in the decision making so ultimately, a discussion between her and her provider is necessary to determine whether she is a good candidate for TOLAC or not. Many women still attempt TOLACs and with great success so long as they are good candidates in a safe medical setting.
Most women should start screening mammograms at age 40 or 10 years younger than the youngest affected family member. With your specific family history however, it is possible that your family may carry a gene mutation which increases your risk of developing cancer beyond that of the general population. I would recommend genetic testing for either your mother or grandmother and then specific screening recommendations for you based on their results. If your primary care physician or Ob/Gyn is not familiar with these tests or guidelines then feel free to come in for a consultation and I will gladly assist you.