How does demyelination in midbrain & medulla impact balance & walking? my relative has limited right leg mobility and can't wiggle toes in both feet. but her lumbar spine mri shows no abnormalities?

Yes, this condition can cause paraplegia (lower extremity paralysis) or quadriplegia (paralysis of all extremities) with negative lumbar spine MRI.

Presume she has been diagnosed with multiple sclerosis or neuromyelitis optica. The lesions over the brainstem do explain problems in the legs. She may have had a relapse or possesses progressive disease. Would make certain that she is taking a potent and appropriate disease modifying therapy. Physical therapy may be useful.