MB here. Just be firm, don't get dragged into the weeks of specific details, and focus on how grateful you are for her assistance these years, what you plan to do to help her find her next position, when her last day will be (set a specific date and stick to it - giving her plenty of notice but also a firm deadline), and what you hope the future relationship will allow.
Don't let concern for her cloud what you know to be the right decision for your family in the long run, even if it's a little tough for the next few weeks.
Good luck.