Hi and welcome,
You are right that MS isnt hereditary, its much more common to be the only person in your family with MS but having said that, some families do actually have multiple living with MS...
Throughout the years i;ve been here i can only recall 1 community member, i believe he had 2 other family members diagnosed so 3 in total. I think his family circumstance would be extra rare though, not just because there's 3 in one family but because of it also being 2 brothers and MS is not as common in men.
Honestly would be thousands of people asking questions because they're experiencing neurological issues and they have a first or second generational diagnosis in their family so they're going through the diagnostic process and needing information and support....most are not diagnosed with MS or even demyelinating disease as you have been, and with MS being the most common type of demyelinating disease, even without your mother's MS diagnosis, MS would be very high on your list of probable causes.
I dont know if this will help keep your thoughts open minded or not but "Demyelination is often secondary to an infectious, an ischemic, a metabolic, or a hereditary disorder or to a toxin (eg, alcohol, ethambutol).. "
"...In primary demyelinating disorders, cause is unknown, but an autoimmune mechanism is suspected because the disorder sometimes follows a viral infection or viral vaccination." [merckmanuals.com overview of Demyelinating disorders]
Lhermitte's Sign is associated with the spinal cord, in neurological conditions like MS it's caused by spinal cord lesions but MS isn't the only causation, i'm not a 100% sure so don't quote me but i think structural damage to the spine itself like degenerative disc disease are the most common causes.
Hope that helps.......JJ
PS if i can help in any way please just ask and i will do my best to get you the right answer!