Anyone choose PBMX who was BRCA neg. with a BRCA + family member

The situation you describe doesn't seem to me to be one that would warrant a PBMX.  Quite the opposite, in fact.

If someone in your family is BRCA+, then you know the very specific genetic mutation that affects your family and that you possibly could have inherited.  Once you are tested for that very specific mutation, if you test negative, you are in the clear.  That's the best possible result of BRCA testing.  Knowing that there is a genetic mutation in your family that is causing the breast cancer, and knowing that you don't have that specific mutation means that your risk should be no different than the average woman.

Once somebody in the family has tested positive, the testing of other individuals in the family is quicker and less expensive.  This is because instead of having to run through and check for all the hundreds (or is it thousands?) of possible BRCA mutations, the specific mutation that is in that family has already been identified. That's the only mutation that needs to be checked for.  BRCA mutations are inherited; someone can't have a BRCA mutation that isn't already in the family.  So this means that the fact that there are likely many mutations yet to be discovered doesn't matter in this type of situation.  The BRCA mutation affecting the family has already been identified by virtue of the fact that someone in the family tested positive.

I would think that a PBMX would be more appropriate for someone who has a significant family history of breast cancer, and yet everyone who's had cancer has been tested and found to be BRCA negative.  In this sort of situation, it might very well be that there is a genetic cause for the breast cancer, but whatever it is has yet to be discovered.  So not knowing what it is, it's impossible to know if someone might be affected or not.  That would seem to be a situation that is more likely to warrant a PBMX for an individual who is in a family that is full of breast cancer. 

Sorry, I realized from your first post that the situation does not apply to you.  I was trying to write your answer from a general perspective, as it would apply to anyone, but maybe that didn't come across.

I had BRCA testing and tested negative.  My genetic counsellor explained that it's highly likely that there are BRCA1 and BRCA2 mutations that have not yet been discovered.  It's also highly likely that there area other genetic mutations, also not yet discovered, that also increase the risk of breast cancer.  And there are some other genetic mutations that are known that also increase breast cancer risk.  That's all very true.

But in the situation you described in your original post, that's all likely not going to matter.  If there is BC in someone's family and the specific mutation that's causing the BC has been identified, then the fact that there may be undiscovered mutations really doesn't matter.  Someone in that family either has the known mutation and is high risk, or they don't have it and are average risk.

The undiscovered mutations and other genes matter to those of us who have families where BC is common but no known mutation has been found in the family.  In that case, one of these undiscovered mutations might be the cause.