Hi Terry, of course you're welcome. We have many who are here because of a loved one having breast cancer.
Laurabelle has recommended this site before:
http://www.facingourrisk.org/ I'm sure she'll chime in as she's knowledgeable about this.
I was tested this past summer, but I've already had breast cancer. They felt I should have my ovaries out if I tested positive. I was negative, but that in itself was good information for me and the rest of my family to know.
Who are you working with on this? You must have met with a genetic counselor, right? Does your sister have an oncologist that she's working with? I would think you would really have to have guidance from an oncologist here as they are the experts in all things cancer and risk. I first sat down with my oncologist and we talked at lenght about my family history and why it might be important
healthwise to be tested. Then I met with the genetic counselor and talked with her at length about the
implications of testing. I'd already decided if I was positive, I'd have my ovaries out. One of the questions I asked my onc before testing was "can I have them out anyway" (which would eliminate the complexities of testing but presumably take care of the problem) and she said yes, and it would be fully covered.
So I guess you have a decision to make re whether to have prophylactic mastectomies and oophorectomy if you test positive. Have they mentioned the possiblity of starting you on tamoxifin if you did not want to go as far as mastectomy (this is sometimes done even in the case of actually having precancerous breast cells, though you'd still have the ovarian issue). I have a friend whose mother died of breast cancer at a young age and I've mentioned testing to her with the thought of possibly prophylactic surgery, and she won't hear of it. I've thought a lot about what I might do in that same situation - knowing what I now know and having been through the heartache of having cancer - and I think that I might very well look into the same thing you are. (I'm sure Christina's story inspired a lot of people, bless her for sharing it publicly.) You see what your sister's been through. Hopefully we're all going to be ok long term (things are certainly looking better with this over the past decade or so), but still, if it could be avoided, then that is something that should be given great thought.
I commend you for going forward with testing and tackling the issue head on. It's definitely not an easy one. Good luck with your decision. I know there are others here who have been tested, but I believe most have been in the setting of actually having cancer. There may be others who have family members that have. I'm sure others will chime in with additional thoughts.

(Let us know how you make out.)