What is "High Risk"?

Not likely.  1 out of every 3 women will be diagnosed with cancer, and one out of every 8 will be diagnosed with breast cancer.  So having 2 cases of BC in one family over several generations is not a sign for concern, unless these happened to be the only women in your family (i.e. if only two women have had BC but if the family is almost entirely male and they are the only two women in the family over 3 generations, that might say something... but then you'd be looking for cases of prostate cancer among the men).  Most cases of breast cancer are random and not genetic.  

One of the signs of a possible genetic cause is if the diagnosis comes when someone is younger and particularly, pre-menopausal.   It sounds as though that wasn't the case with either your mother or your great aunt.  So in your case, with only one 1st degree relative with breast cancer (your mother) and with her having a post-menopausal diagnosis, that's not likely to increase your risk by much, if by anything at all. Your great aunt is too far removed from you to confer any risk, unless she was one of a line of women in the family who'd all been diagnosed with breast cancer (or ovarian cancer).  As you can see from this risk calculator, it's only first degree relatives that they ask about:  http://www.cancer.gov/bcrisktool/

Also, you may find our section on "Understanding Breast Cancer Risk and How to Lower It", helpful!

http://www.breastcancer.org/risk/understand

and this too:

http://www.breastcancer.org/risk