Here is the reply I have just sent:
Hi Aunt Fern,
Yes, I am aware of the things you mentioned about living in Country A. However, the company we are applying with only operates in Country A, so Country B is not going to be possible except, perhaps, for a visit. We are very definite in our choice of this particular company, most notably, their great record and reputation among their expat work force.
(I did go on at this point to discuss some other things DH and I have on the horizon, more or less to soften the blow of rejecting her expertise, but none of those things are pertinent to the issue of our potential move).