Well, Tumbleweed, you're a good brother in-law to be concerned!
I never had a migraine until I began the perimenopause years. I thought it was my eyes at first, because I couldn't be in any kind of bright light, but, just like Lisa mentioned, they happened the same time every month. What a coincidence! The gyn told me they were common with menopause, and thereafter I called them my hormone headaches. Now I only get them occasionnally, usually when the weather changes and I have a sinus attack. So, now I call them my sinus headaches....
I haven't. so far, had headaches I'd associate with my AN, altho for the first time ever last year I had an "aura" (w/o the headache!) and thought it was my eyes again ...
However, from what you said, your sister-in-law's headaches are very bad, and seem to be getting worse. She should pursue with her doctor to make sure that it's not anything more serious than those hormones fluctuating.
I hope for the best for her.
Elaine