I presented with migraines too. No eye problems such as losing vision.
Thing is, years earlier I had perfect vision. Suddenly that changed - and I mean one week I had an eye test, 20/20 vision, next week I needed glasses. My neuro asked what was going on at the time. I was working in really hot temps, and he thinks that may have been an early sign of ms.
My first MRI showed brain lesions, but all the other tests, no sign of ms.
I had C spine, T spine lumbar MRIS, EMG, EKG, EEG, blood work - no ms, ruled every normal mimic out. But I was dx'd with C spine disorders and a cyst in the T spine. 2 possible mimics, but not likely migraine makers. Hmmm.
My neuro said it can take years for ms tests to come out positive. But there are so many mimics it's scary. It could be a lot of other things. Hopefully, your SIL has a good team of docs who will keep on this until they figure it out for her.