Hi and welcome,
I'm not really sure where you've got MS as a possible explanation for your symptoms, the types of common symptoms associated with dehydration don't usually mimic close enough to be suggestive of MS, eg dehydration = lightheaded dizziness vs MS = vertigo.
Vit D deficientcy is actually very common, well here in oz it is and we're a sunny country, our stats are 1 in 3 are deficient so whilst vit D has been associated with MS as well as a lot of other conditions, that in itself wouldn't be suggestive of MS...
Again i'm not really sure where you're saying, white blood cell isn't suppose to be low for MS if that's what you mean, no blood test results are relevant for MS yet though last i read they are getting close but it's nothing to do with white blood cells count from my understanding.
Technically MS is not a genetically inherited condition, with a first person family relation eg mother, father, sister, brother the odds are only slightly increases but the further away the person is to you the more you are genetically different so the significance of their MS dx becomes even less relevant to you.
Off the top of my head [so don't quote me :D] i think with you having a grandparent dx with MS is doesn't really increase the genetic connection, its only something like 3 -5% of grandchildren also dx with MS or in other words a grandchild won't be dx with MS over 95% of the time. It's still more common to be the only person in the family dx with MS, than there to be multiple, though that does happen in some families though it's less common.
What type of symptoms are you experiencing that you think might be suggestive of MS?
Hope that helps......JJ